diff --git a/app/layout.tsx b/app/layout.tsx
index 7700f45..de124e3 100644
--- a/app/layout.tsx
+++ b/app/layout.tsx
@@ -11,6 +11,7 @@ import { SubpageNav } from '@/components/SubpageNav';
 import { ThemeToggle } from '@/components/ThemeToggle';
 import { ScrollToTop } from '@/components/ScrollToTop';
 import { Footer } from '@/components/Footer';
+import { DevToolbar } from '@/components/DevToolbar';
 import { getConfig, getGoogleFontsUrl } from '@/lib/config';
 
 const siteTitle = process.env.SITE_TITLE || 'Gallery';
@@ -83,6 +84,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
         <main className="main">{children}</main>
         <Footer />
         <ScrollToTop />
+        {process.env.NODE_ENV === 'development' && <DevToolbar />}
       </body>
     </html>
   );
diff --git a/components/DevToolbar.tsx b/components/DevToolbar.tsx
new file mode 100644
index 0000000..0c1ec27
--- /dev/null
+++ b/components/DevToolbar.tsx
@@ -0,0 +1,445 @@
+/**
+ * DevToolbar — floating visual builder for theme previewing.
+ *
+ * Only renders when NODE_ENV === 'development'.
+ * Modifies data-* attributes on <html> to live-preview themes,
+ * photo frames, dark/light mode, and grain without a rebuild.
+ *
+ * Hero styles and grid layouts require server re-render, so
+ * those sections show a copyable YAML snippet instead.
+ */
+
+'use client';
+
+import { useState, useEffect, useCallback } from 'react';
+
+const PRESETS = ['studio', 'minimal', 'editorial', 'classic', 'noir', 'monograph', 'botanica'];
+const FRAMES = ['none', 'passepartout', 'shadow'];
+const HERO_STYLES = ['split', 'fullbleed', 'minimal', 'stacked', 'typographic', 'mosaic'];
+const GRID_LAYOUTS = ['masonry', 'uniform', 'showcase', 'filmstrip', 'editorial-flow'];
+
+// Font stacks for each preset (must match THEME_PRESETS in config.ts)
+const PRESET_FONTS: Record<string, { heading: string; body: string; caption: string }> = {
+    studio: { heading: 'Playfair Display', body: 'DM Sans', caption: 'EB Garamond' },
+    minimal: { heading: 'Geist', body: 'Geist', caption: 'IBM Plex Mono' },
+    editorial: { heading: 'Bodoni Moda', body: 'Newsreader', caption: 'IBM Plex Mono' },
+    classic: { heading: 'Cinzel', body: 'Crimson Pro', caption: 'EB Garamond' },
+    noir: { heading: 'Libre Baskerville', body: 'Source Sans 3', caption: 'IBM Plex Mono' },
+    monograph: { heading: 'Instrument Serif', body: 'Inter', caption: 'JetBrains Mono' },
+    botanica: { heading: 'Cormorant Garamond', body: 'Nunito Sans', caption: 'Lora' },
+};
+
+const PRESET_ACCENTS: Record<string, string> = {
+    studio: '#e60012',
+    minimal: '#000000',
+    editorial: '#8B2500',
+    classic: '#c49a3c',
+    noir: '#ff6b35',
+    monograph: '#333333',
+    botanica: '#4a7c59',
+};
+
+function getAttr(attr: string): string {
+    return document.documentElement.getAttribute(attr) ?? '';
+}
+
+function setAttr(attr: string, value: string) {
+    document.documentElement.setAttribute(attr, value);
+}
+
+function setVar(name: string, value: string) {
+    document.documentElement.style.setProperty(name, value);
+}
+
+export function DevToolbar() {
+    const [open, setOpen] = useState(false);
+    const [preset, setPreset] = useState('');
+    const [frame, setFrame] = useState('');
+    const [theme, setTheme] = useState('');
+    const [grain, setGrain] = useState('');
+    const [copiedYaml, setCopiedYaml] = useState('');
+    const [gridCols, setGridCols] = useState(3);
+    const [gridGap, setGridGap] = useState(12);
+
+    // Read initial values from DOM
+    useEffect(() => {
+        setPreset(getAttr('data-preset'));
+        setFrame(getAttr('data-photo-frame'));
+        setTheme(getAttr('data-theme') || 'dark');
+        setGrain(getAttr('data-grain'));
+        // Read grid vars
+        const style = getComputedStyle(document.documentElement);
+        const cols = parseInt(style.getPropertyValue('--grid-columns') || '3', 10);
+        const gap = parseInt(style.getPropertyValue('--grid-gap') || '12', 10);
+        if (!isNaN(cols)) setGridCols(cols);
+        if (!isNaN(gap)) setGridGap(gap);
+    }, []);
+
+    const switchPreset = useCallback((p: string) => {
+        setAttr('data-preset', p);
+        setPreset(p);
+        // Update accent + fonts
+        const accent = PRESET_ACCENTS[p] || '#e60012';
+        setVar('--accent', accent);
+        setVar('--accent-dim', `${accent}1f`);
+        const fonts = PRESET_FONTS[p];
+        if (fonts) {
+            setVar('--font-serif', `'${fonts.heading}', Georgia, 'Times New Roman', serif`);
+            setVar('--font-sans', `'${fonts.body}', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif`);
+            setVar('--font-caption', `'${fonts.caption}', Georgia, serif`);
+            // Dynamically load Google Fonts if not already loaded
+            const families = [fonts.heading, fonts.body, fonts.caption]
+                .filter((f, i, a) => a.indexOf(f) === i)
+                .map((f) => `family=${f.replace(/\s+/g, '+')}:ital,wght@0,300;0,400;0,500;0,600;0,700;1,400`)
+                .join('&');
+            const href = `https://fonts.googleapis.com/css2?${families}&display=swap`;
+            // Check if already loaded
+            const existing = document.querySelector(`link[href="${href}"]`);
+            if (!existing) {
+                const link = document.createElement('link');
+                link.rel = 'stylesheet';
+                link.href = href;
+                document.head.appendChild(link);
+            }
+        }
+    }, []);
+
+    const switchFrame = useCallback((f: string) => {
+        setAttr('data-photo-frame', f);
+        setFrame(f);
+    }, []);
+
+    const switchTheme = useCallback((t: string) => {
+        setAttr('data-theme', t);
+        setTheme(t);
+    }, []);
+
+    const toggleGrain = useCallback(() => {
+        const next = grain === 'true' ? 'false' : 'true';
+        setAttr('data-grain', next);
+        setGrain(next);
+    }, [grain]);
+
+    const updateGridCols = useCallback((cols: number) => {
+        setGridCols(cols);
+        // Update all photo-grid elements
+        document.querySelectorAll<HTMLElement>('.photo-grid').forEach((el) => {
+            el.style.setProperty('--grid-columns', String(cols));
+        });
+    }, []);
+
+    const updateGridGap = useCallback((gap: number) => {
+        setGridGap(gap);
+        document.querySelectorAll<HTMLElement>('.photo-grid').forEach((el) => {
+            el.style.setProperty('--grid-gap', `${gap}px`);
+        });
+    }, []);
+
+    const copyYaml = useCallback(
+        (section: string, yaml: string) => {
+            navigator.clipboard.writeText(yaml);
+            setCopiedYaml(section);
+            setTimeout(() => setCopiedYaml(''), 1500);
+        },
+        [],
+    );
+
+    const currentYaml = `theme:
+  preset: ${preset}
+  photoFrame: ${frame}
+  grain: ${grain === 'true'}`;
+
+    // Keyboard shortcut: Ctrl/Cmd + Shift + T
+    useEffect(() => {
+        const handler = (e: KeyboardEvent) => {
+            if ((e.metaKey || e.ctrlKey) && e.shiftKey && e.key === 'T') {
+                e.preventDefault();
+                setOpen((o) => !o);
+            }
+        };
+        document.addEventListener('keydown', handler);
+        return () => document.removeEventListener('keydown', handler);
+    }, []);
+
+    return (
+        <>
+            {/* ── Floating Toggle Button ─────────────────────── */}
+            <button
+                onClick={() => setOpen(!open)}
+                aria-label="Dev Toolbar"
+                style={{
+                    position: 'fixed',
+                    bottom: 20,
+                    left: 20,
+                    zIndex: 99999,
+                    width: 44,
+                    height: 44,
+                    borderRadius: '50%',
+                    border: '2px solid rgba(255,255,255,0.2)',
+                    background: open
+                        ? 'linear-gradient(135deg, #f43f5e, #a855f7)'
+                        : 'rgba(30,30,30,0.9)',
+                    backdropFilter: 'blur(12px)',
+                    color: '#fff',
+                    fontSize: 18,
+                    cursor: 'pointer',
+                    display: 'flex',
+                    alignItems: 'center',
+                    justifyContent: 'center',
+                    transition: 'all 0.2s ease',
+                    boxShadow: '0 4px 24px rgba(0,0,0,0.4)',
+                }}
+            >
+                {open ? '✕' : '🎨'}
+            </button>
+
+            {/* ── Panel ──────────────────────────────────────── */}
+            {open && (
+                <div
+                    style={{
+                        position: 'fixed',
+                        bottom: 76,
+                        left: 20,
+                        zIndex: 99998,
+                        width: 300,
+                        maxHeight: 'calc(100vh - 120px)',
+                        overflowY: 'auto',
+                        background: 'rgba(18,18,22,0.96)',
+                        backdropFilter: 'blur(20px)',
+                        borderRadius: 16,
+                        border: '1px solid rgba(255,255,255,0.1)',
+                        boxShadow: '0 8px 48px rgba(0,0,0,0.6)',
+                        color: '#e4e4e7',
+                        fontFamily: '-apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif',
+                        fontSize: 12,
+                        padding: 0,
+                    }}
+                >
+                    {/* ── Header ──────────────────────────── */}
+                    <div
+                        style={{
+                            padding: '14px 16px 10px',
+                            borderBottom: '1px solid rgba(255,255,255,0.08)',
+                            display: 'flex',
+                            alignItems: 'center',
+                            justifyContent: 'space-between',
+                        }}
+                    >
+                        <span style={{ fontWeight: 700, fontSize: 13, letterSpacing: '0.02em' }}>
+                            🎨 Theme Builder
+                        </span>
+                        <span
+                            style={{
+                                fontSize: 10,
+                                color: '#71717a',
+                                background: 'rgba(255,255,255,0.06)',
+                                padding: '2px 8px',
+                                borderRadius: 6,
+                            }}
+                        >
+                            DEV ONLY
+                        </span>
+                    </div>
+
+                    {/* ── Preset ──────────────────────────── */}
+                    <Section title="Preset">
+                        <PillGrid
+                            items={PRESETS}
+                            active={preset}
+                            onSelect={switchPreset}
+                        />
+                    </Section>
+
+                    {/* ── Dark / Light ────────────────────── */}
+                    <Section title="Mode">
+                        <PillGrid
+                            items={['dark', 'light']}
+                            active={theme}
+                            onSelect={switchTheme}
+                        />
+                    </Section>
+
+                    {/* ── Photo Frame ─────────────────────── */}
+                    <Section title="Photo Frame">
+                        <PillGrid
+                            items={FRAMES}
+                            active={frame}
+                            onSelect={switchFrame}
+                        />
+                    </Section>
+
+                    {/* ── Grain ───────────────────────────── */}
+                    <Section title="Film Grain">
+                        <button
+                            onClick={toggleGrain}
+                            style={{
+                                ...pillStyle,
+                                background: grain === 'true'
+                                    ? 'linear-gradient(135deg, #f43f5e, #a855f7)'
+                                    : 'rgba(255,255,255,0.06)',
+                                color: grain === 'true' ? '#fff' : '#a1a1aa',
+                                border: grain === 'true' ? '1px solid rgba(255,255,255,0.2)' : '1px solid rgba(255,255,255,0.06)',
+                            }}
+                        >
+                            {grain === 'true' ? '● On' : '○ Off'}
+                        </button>
+                    </Section>
+
+                    {/* ── Grid Controls ───────────────────── */}
+                    <Section title="Grid">
+                        <div style={{ display: 'flex', gap: 12, alignItems: 'center' }}>
+                            <label style={{ color: '#71717a', fontSize: 11, minWidth: 45 }}>Cols: {gridCols}</label>
+                            <input
+                                type="range"
+                                min={1}
+                                max={6}
+                                value={gridCols}
+                                onChange={(e) => updateGridCols(Number(e.target.value))}
+                                style={{ flex: 1, accentColor: '#a855f7' }}
+                            />
+                        </div>
+                        <div style={{ display: 'flex', gap: 12, alignItems: 'center', marginTop: 8 }}>
+                            <label style={{ color: '#71717a', fontSize: 11, minWidth: 45 }}>Gap: {gridGap}px</label>
+                            <input
+                                type="range"
+                                min={0}
+                                max={32}
+                                value={gridGap}
+                                onChange={(e) => updateGridGap(Number(e.target.value))}
+                                style={{ flex: 1, accentColor: '#a855f7' }}
+                            />
+                        </div>
+                    </Section>
+
+                    {/* ── Hero Style (info only) ──────────── */}
+                    <Section title="Hero Style (server)">
+                        <PillGrid items={HERO_STYLES} active="" onSelect={(h) => {
+                            copyYaml('hero', `theme:\n  heroStyle: ${h}`);
+                        }} />
+                        {copiedYaml === 'hero' && <CopyBadge />}
+                    </Section>
+
+                    {/* ── Grid Layout (info only) ─────────── */}
+                    <Section title="Grid Layout (server)">
+                        <PillGrid items={GRID_LAYOUTS} active="" onSelect={(l) => {
+                            copyYaml('layout', `grid:\n  layout: ${l}`);
+                        }} />
+                        {copiedYaml === 'layout' && <CopyBadge />}
+                    </Section>
+
+                    {/* ── Export YAML ──────────────────────── */}
+                    <div
+                        style={{
+                            padding: '12px 16px 16px',
+                            borderTop: '1px solid rgba(255,255,255,0.08)',
+                        }}
+                    >
+                        <button
+                            onClick={() => copyYaml('all', currentYaml)}
+                            style={{
+                                width: '100%',
+                                padding: '10px 0',
+                                borderRadius: 10,
+                                border: 'none',
+                                background: copiedYaml === 'all'
+                                    ? '#22c55e'
+                                    : 'linear-gradient(135deg, #a855f7, #6366f1)',
+                                color: '#fff',
+                                fontWeight: 600,
+                                fontSize: 12,
+                                cursor: 'pointer',
+                                transition: 'all 0.15s ease',
+                                letterSpacing: '0.02em',
+                            }}
+                        >
+                            {copiedYaml === 'all' ? '✓ Copied to clipboard' : 'Copy YAML to clipboard'}
+                        </button>
+                    </div>
+                </div>
+            )}
+        </>
+    );
+}
+
+/* ── Sub-components ───────────────────────────────── */
+
+function Section({ title, children }: { title: string; children: React.ReactNode }) {
+    return (
+        <div style={{ padding: '12px 16px 8px' }}>
+            <div
+                style={{
+                    fontSize: 10,
+                    fontWeight: 600,
+                    textTransform: 'uppercase',
+                    letterSpacing: '0.1em',
+                    color: '#71717a',
+                    marginBottom: 8,
+                }}
+            >
+                {title}
+            </div>
+            {children}
+        </div>
+    );
+}
+
+const pillStyle: React.CSSProperties = {
+    padding: '6px 12px',
+    borderRadius: 8,
+    fontSize: 11,
+    fontWeight: 500,
+    cursor: 'pointer',
+    transition: 'all 0.12s ease',
+    whiteSpace: 'nowrap',
+};
+
+function PillGrid({
+    items,
+    active,
+    onSelect,
+}: {
+    items: string[];
+    active: string;
+    onSelect: (item: string) => void;
+}) {
+    return (
+        <div style={{ display: 'flex', flexWrap: 'wrap', gap: 6 }}>
+            {items.map((item) => (
+                <button
+                    key={item}
+                    onClick={() => onSelect(item)}
+                    style={{
+                        ...pillStyle,
+                        background:
+                            item === active
+                                ? 'linear-gradient(135deg, #a855f7, #6366f1)'
+                                : 'rgba(255,255,255,0.06)',
+                        color: item === active ? '#fff' : '#a1a1aa',
+                        border:
+                            item === active
+                                ? '1px solid rgba(255,255,255,0.2)'
+                                : '1px solid rgba(255,255,255,0.06)',
+                    }}
+                >
+                    {item}
+                </button>
+            ))}
+        </div>
+    );
+}
+
+function CopyBadge() {
+    return (
+        <span
+            style={{
+                display: 'inline-block',
+                marginTop: 6,
+                fontSize: 10,
+                color: '#22c55e',
+                fontWeight: 500,
+            }}
+        >
+            ✓ YAML copied — paste into gallery.yaml
+        </span>
+    );
+}