feat(theme): improvements and refactoring of theme module

fix: theme rebase

src/app/cosmic.rs

@@ -156,7 +156,7 @@ where
     }
 
     fn theme(&self) -> Self::Theme {
-        THEME.with(|t| t.borrow().clone())
+        crate::theme::active()
     }
 
     #[cfg(feature = "wayland")]

src/app/settings.rs

@@ -91,7 +91,7 @@ impl Default for Settings {
             size: (1024, 768),
             #[cfg(feature = "wayland")]
             size_limits: Limits::NONE.min_height(1.0).min_width(1.0),
-            theme: crate::theme::theme(),
+            theme: crate::theme::system_preference(),
             transparent: false,
         }
     }