servoshell: Ensure that the theme is applied properly on startup (#37128)

Instead of only applying the theme when it changes, apply it to both
egui and all
newly created `WebView`s. 

Reference Zulip thread:
https://servo.zulipchat.com/#narrow/channel/437943-embedding/topic/Theme.20.28light.2Fdark.20mode.29.20support.20on.20initial.20page.20load/with/520082314

Testing: There are tests for servoshell currently, so all testing is
manual.
Fixes: #34913

Signed-off-by: Tony <legendmastertony@gmail.com>

ports/servoshell/desktop/window_trait.rs

@@ -48,6 +48,8 @@ pub trait WindowPortsMethods {
         _position: servo::webrender_api::units::DeviceIntRect,
     ) {
     }
-
     fn hide_ime(&self) {}
+    fn theme(&self) -> servo::Theme {
+        servo::Theme::Light
+    }
 }