Function api_arborium_theme_css 

pub async fn api_arborium_theme_css() -> impl IntoResponse

Serves arborium syntax highlighting CSS for both light and dark modes.

Emits CSS custom properties (--hl-{tag}) in :root for each theme, then a single set of element rules referencing those variables. This lets other UI styles reuse --hl-keyword, --hl-function, etc.