Settings → Appearance.
The eight themes
Four dark: Moonpool (moonlit slate and silver-periwinkle, the default), Midnight Library (warm wood and lamplight), Abyssal Calm (deep navy and teal), VOID_SIGNAL (phosphor-on-black terminal). Four light: Daybreak (warm paper and copper ink), Shorelight, Matinee (1984 beige-box computing, scanlines included), and Atelier.
A theme here is not a palette swap. Each one carries its own typography, its own motion feel, and its own atmosphere, which is why switching feels like a different room rather than a different coat of paint. Every surface in the app works in all eight; that is a standing engineering rule, not a hope.
Your choice is saved to your account and applies before first paint, so there is no flash of the wrong theme on load.
The character
The Character tab picks who Margin looks like: the small living face that sits with the assistant and reacts while it works. Same assistant either way; this is purely about who you would rather share a desk with.
Reduced motion
The app honours your system's reduced-motion preference: entrances settle instantly and loops hold still. If motion feels heavy on an older machine, the OS-level switch is the intended control.
For the curious
The landing page has its own theme toggle (Daybreak and Moonpool only) that is deliberately separate from the app: trying a theme on the marketing site never touches your saved preference.