Add custom theme colors, full per-token control (#30)

Adds a 5th "Custom" option to the theme swatch grid alongside the
existing 4 presets, opening a picker for all ~12 CSS custom properties
(backgrounds, borders, text, three accent tiers, highlight, danger) plus
a light/dark toggle for native control rendering.

Persisted as a new users.custom_theme_colors JSONB column, validated
server-side against exactly what a native <input type="color"> can ever
produce. Colors survive switching to a preset and back, since there's no
reason picking a preset for a moment should force redoing every color
pick. Applied at runtime as inline custom properties on :root (presets
stay static CSS) via a shared applyTheme() helper, which is also
responsible for clearing those inline overrides when switching away --
otherwise they'd silently keep winning over whatever preset's own
stylesheet values should apply next.

Live preview on every color change; persists only on explicit "Save
colors" (not per keystroke, since a color input fires continuously while
dragging), and closing the modal without saving reverts the preview back
to whatever's actually persisted.

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

backend/app/schemas/user.py

@@ -11,6 +11,31 @@ class UserCreate(BaseModel):
     password: str = Field(min_length=8, max_length=200)
 
 
+# Matches exactly the CSS custom properties frontend/src/styles/themes.css
+# overrides per built-in preset -- a "custom" theme is applied the same way,
+# just as inline styles on :root instead of a static stylesheet block (see
+# frontend/src/lib/theme.ts). Hex-only (`#rrggbb`) since that's exactly what
+# a native <input type="color"> always produces -- no alpha, no shorthand --
+# so the pattern constraint can't reject anything the picker UI itself sends.
+_HEX_COLOR = Field(pattern=r"^#[0-9a-fA-F]{6}$")
+
+
+class CustomThemeColors(BaseModel):
+    void: str = _HEX_COLOR
+    void_2: str = _HEX_COLOR
+    surface: str = _HEX_COLOR
+    surface_2: str = _HEX_COLOR
+    border: str = _HEX_COLOR
+    text: str = _HEX_COLOR
+    muted: str = _HEX_COLOR
+    accent: str = _HEX_COLOR
+    accent_2: str = _HEX_COLOR
+    accent_3: str = _HEX_COLOR
+    highlight: str = _HEX_COLOR
+    danger: str = _HEX_COLOR
+    color_scheme: Literal["light", "dark"]
+
+
 class UserRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
@@ -21,6 +46,7 @@ class UserRead(BaseModel):
     is_site_admin: bool
     display_name: str | None
     theme: str | None
+    custom_theme_colors: CustomThemeColors | None
     avatar_filename: str | None
     appear_offline: bool
     created_at: datetime
@@ -34,7 +60,8 @@ class ProfileUpdate(BaseModel):
     # their defaults, and vice versa.
     display_name: str | None = Field(default=None, max_length=50)
     # Kept in sync with frontend/src/styles/themes.css's theme blocks.
-    theme: Literal["dark", "light", "midnight", "sunset"] | None = Field(default=None)
+    theme: Literal["dark", "light", "midnight", "sunset", "custom"] | None = Field(default=None)
+    custom_theme_colors: CustomThemeColors | None = Field(default=None)
     appear_offline: bool | None = Field(default=None)