in src/platform/unix.rs [476:496]
fn element_color(&self, element: Element, window_active: bool) -> ARGBColor;
/// Color for a given button part.
fn button_color(
&self,
button: Button,
state: ButtonState,
foreground: bool,
window_active: bool,
) -> ARGBColor;
/// Font name and the size for the title bar.
///
/// By default the font is `sans-serif` at the size of 11.
///
/// Returning `None` means that title won't be drawn.
fn font(&self) -> Option<(String, f32)> {
// Not having any title isn't something desirable for the users, so setting it to
// something generic.
Some((String::from("sans-serif"), 11.))
}