Preferences card theme now applies instantly

Picking a theme in the Preferences card on your account page now applies it the moment you pick it, the same behavior as the sidebar user menu's Theme submenu. Until this change, the card waited for you to click Save before switching.

Language and time zone in the same card are unaffected: they still apply on Save. Theme also still isn't saved to your account: it's a browser preference, not one that follows you across devices.