themes get their own control, and the vimazing keys

Fourteen themes was too many to keep under an avatar next to "Sign out" — that is where
a preference goes when nobody has decided it matters. It now has its own button in the
header, between the controls and the avatar, grouped by family.

── The keys, ported from the owner's vimazing ──

  ctrl+shift+l / ctrl+shift+h   cycle forward / back, from anywhere, no UI
  ctrl+alt+space                open the list
  j / k                         move the focus
  l or space                    apply the focused theme to the app BEHIND the list
  h                             back to where you started
  enter                         keep what you are looking at
  escape                        restore and close

The browse is the point, and it is why the overlay is a portal over a 55% backdrop
rather than a modal: you are not reading names, you are watching the application change.
The theme you started on is captured when the list opens, so `h` and `escape` undo a
whole browse however many previews happened inside it.

Typing is still typing: the list ignores j/k/l/h while focus is in an input, textarea,
select or anything contenteditable.

── Where it lives ──

The key handling is in the theme package — it is behaviour that belongs to themes, and
it needs no UI. The list and the button are Officer's, because a dialog would drag a
component library into a package that currently has none.

Verified by driving the real keyboard in a browser:

  start duck-dark · ctrl+shift+l -> plain · ctrl+alt+space -> overlay
  j, l -> tokyo-night (previewed) · escape -> plain, closed

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/workspaces/theme/src/index.ts

@@ -6,5 +6,6 @@
  */
 export { ThemeProvider, useTheme, useColorMode } from './ThemeProvider';
 export { useThemeAsset } from './useThemeAsset';
+export { useThemeKeys } from './useThemeKeys';
 export { THEMES, DEFAULT_THEME, findTheme, isKnownTheme, isDarkTheme, themeFromLegacyColorMode } from './themes';
 export type { Theme, ThemeId } from './themes';

src/workspaces/theme/src/useThemeKeys.ts

@@ -0,0 +1,126 @@
+import { useCallback, useEffect, useRef, useState } from 'react';
+import { useTheme } from './ThemeProvider';
+import type { ThemeId } from './themes';
+
+/**
+ * Keyboard theme switching, ported from the owner's `vimazing`.
+ *
+ * Two modes, and the second is the good one:
+ *
+ *   ctrl+shift+l / ctrl+shift+h   cycle forward / back, from anywhere, no UI
+ *   ctrl+alt+space                open the list, then browse it with j/k
+ *
+ * ── Why browsing applies the theme ──
+ *
+ * With the list open, `j`/`k` move the focus and `l` or `space` APPLIES the focused theme to the whole
+ * app behind the overlay. That is the point of it: you are not reading names, you are watching the
+ * application change. `h` puts back what you started with, `Enter` keeps what you are looking at, `Escape`
+ * restores and closes.
+ *
+ * The starting theme is captured when the list opens, so `h` and `Escape` can undo an entire browse
+ * regardless of how many previews happened in between.
+ *
+ * ── Not a UI ──
+ *
+ * This hook owns the keys and the selection; something else draws the list. It lives in the theme package
+ * because the behaviour belongs to themes, while a dialog would drag a component library in behind it.
+ */
+export function useThemeKeys() {
+  const { themeId, setTheme, themes } = useTheme();
+  const [open, setOpen] = useState(false);
+  const [focused, setFocused] = useState<ThemeId>(themeId);
+  const startedOn = useRef<ThemeId>(themeId);
+
+  // A ref, so the handler does not need re-binding on every theme change.
+  const state = useRef({ themeId, focused, open });
+  state.current = { themeId, focused, open };
+
+  const step = useCallback(
+    (from: ThemeId, delta: number): ThemeId => {
+      const ids = themes.map((t) => t.id);
+      const at = ids.indexOf(from);
+      return ids[(at + delta + ids.length) % ids.length]!;
+    },
+    [themes],
+  );
+
+  useEffect(() => {
+    const onKeyDown = (event: KeyboardEvent) => {
+      const { themeId: current, focused: focus, open: isOpen } = state.current;
+
+      // Toggle the list.
+      if (event.ctrlKey && event.altKey && event.code === 'Space') {
+        event.preventDefault();
+        setOpen((was) => {
+          if (!was) {
+            startedOn.current = current;
+            setFocused(current);
+          }
+          return !was;
+        });
+        return;
+      }
+
+      // Cycle from anywhere, list or no list.
+      if (event.ctrlKey && event.shiftKey && (event.key === 'L' || event.key === 'l')) {
+        event.preventDefault();
+        setTheme(step(current, 1));
+        return;
+      }
+      if (event.ctrlKey && event.shiftKey && (event.key === 'H' || event.key === 'h')) {
+        event.preventDefault();
+        setTheme(step(current, -1));
+        return;
+      }
+
+      if (!isOpen) return;
+
+      // Typing in a field should still be typing, even with the list up.
+      const target = event.target as HTMLElement | null;
+      if (target && (target.isContentEditable || /^(input|textarea|select)$/i.test(target.tagName))) return;
+
+      switch (event.key) {
+        case 'j':
+        case 'J':
+        case 'ArrowDown':
+          event.preventDefault();
+          setFocused((prev) => step(prev, 1));
+          return;
+        case 'k':
+        case 'K':
+        case 'ArrowUp':
+          event.preventDefault();
+          setFocused((prev) => step(prev, -1));
+          return;
+        case 'l':
+        case 'L':
+        case ' ':
+          // Apply what is focused, without closing — the browse.
+          event.preventDefault();
+          setTheme(focus);
+          return;
+        case 'h':
+        case 'H':
+          event.preventDefault();
+          setTheme(startedOn.current);
+          setFocused(startedOn.current);
+          return;
+        case 'Enter':
+          event.preventDefault();
+          setTheme(focus);
+          setOpen(false);
+          return;
+        case 'Escape':
+          event.preventDefault();
+          setTheme(startedOn.current);
+          setOpen(false);
+          return;
+      }
+    };
+
+    window.addEventListener('keydown', onKeyDown);
+    return () => window.removeEventListener('keydown', onKeyDown);
+  }, [setTheme, step]);
+
+  return { open, setOpen, focused, setFocused };
+}