fix: Use globals instead of thread-locals

Better support for multi-threaded applications,
especially cosmic-comp rendering in parallel on
multiple threads, each potentially accessing
global configurations such as the active theme,
icon_theme and more...

src/widget/segmented_button/widget.rs

@@ -1361,7 +1361,7 @@ where
                 bounds.y = bounds.y + bounds.height - width;
                 bounds.height = width;
 
-                let rad_0 = THEME.with(|t| t.borrow().cosmic().corner_radii.radius_0);
+                let rad_0 = THEME.lock().unwrap().cosmic().corner_radii.radius_0;
                 renderer.fill_quad(
                     renderer::Quad {
                         bounds,