Alternative appearances.
WHAT TO BASE IT ON
Custom properties, redefined per theme.
WHY
They change at runtime, which build-time variables cannot.
WHAT TO RESPECT
The system preference, as the default.
WHAT TO PROVIDE
A control to override it, with the choice remembered.
WHAT TO OFFER
Three options: light, dark, and follow the system.
WHY
Some people want a fixed choice regardless of the system.
WHAT TO AVOID
A flash of the wrong theme on load.
WHAT CAUSES IT
Applying the theme after the page has rendered.
WHAT TO DO ABOUT IT
Apply it before first paint, with a small inline script.
WHAT TO CHECK IN A DARK THEME
Contrast, which is not automatically adequate Images and logos, which may need alternatives Shadows, which do not work the same way Pure black and pure white, which are harsh
WHAT TO PREFER
Near-black and near-white.
WHAT TO TEST
Both themes, fully, including every state.
WHAT NOT TO ASSUME
That inverting colours produces a good dark theme.
