Function window_css_block 

pub fn window_css_block(
    window: &WindowOptions,
    platform: DesktopPlatform,
) -> String

Build the constant <style> block carrying window CSS custom properties.

The block depends only on window configuration and the target platform, so hosts compute it once at startup and reuse it for every rendered document. Always contains the drag-region rules, because backends inject crate::DRAG_REGION_SCRIPT regardless of titlebar style.