Add custom theme support via CSS plugin

[sgjholt]

Summary

• Adds a + CSS button to the theme bar that opens a .css file picker (File System Access API)
• The loaded CSS is injected into a <style> tag and persisted to localStorage (mb-custom-css) — no re-picking needed on reload
• A Custom button (dashed border) appears once a theme is loaded; clicking it re-activates the theme after switching to a built-in one
• clearCustomTheme() is available in the console to reset back to dark
• Missing CSS variables fall back to the dark theme via normal CSS cascade — no validation required
• + CSS button is hidden alongside other File API controls on non-Chrome browsers

CSS plugin format

[data-theme="custom"] {
  --bg: #1e1e2e;
  --accent: #a6e3a1;
  /* any variables — omitted ones fall back to dark theme */
}

Test plan

• Click + CSS, load a .css file with [data-theme="custom"] { --accent: hotpink; } — confirm accent changes
• Reload page — confirm custom theme persists
• Switch to a built-in theme, click Custom — confirm it re-activates
• npm test passes (20 unit tests)
• npm run test:e2e passes (updated theme button count assertion)

🤖 Generated with Claude Code