Fixes #364 Allows users to manually change theme, useful for cases like LibreWolf where it defaults to light-mode irrespective of system settings (with resistFingerprinting enabled). Stores theme as 'light', 'dark', or 'auto' in the database, and dynamically adds/removes the 'dark' class from the HTML. Signed-off-by: TheBananaPancake <did:plc:3ywraszv5eqhnlvzumwju5fa>