Function set_global_css_from_file_with_priority 

pub fn set_global_css_from_file_with_priority<P: AsRef<Path>>(
    path: P,
    priority: u32,
) -> Result<(), Error>

Sets a custom global stylesheet from a file, with the given priority.

If the file doesn’t exist a tracing::error message will be emitted and an std::io::Error will be returned.

The priority can be any value, but GTK includes some that you can use.