Trilium/apps/client/src/stylesheets/theme.css

/* Import the light color scheme.
 * This is the base color scheme, always active and overridden by the dark
 * color scheme stylesheet when necessary. */
@import url(./theme-light.css);

/* Import the dark color scheme when the system preference is set to dark mode */
@import url(./theme-dark.css) (prefers-color-scheme: dark);

:root {
    --theme-style-auto: true;
}