agent: Ensure background color is the same even while zoomed in (#30804)

Release Notes:

- agent: Fixed the background color of the agent panel changing if you
zoomed it in.

crates/agent/src/active_thread.rs

@@ -3417,6 +3417,7 @@ impl Render for ActiveThread {
         v_flex()
             .size_full()
             .relative()
+            .bg(cx.theme().colors().panel_background)
             .on_mouse_move(cx.listener(|this, _, _, cx| {
                 this.show_scrollbar = true;
                 this.hide_scrollbar_later(cx);

crates/agent/src/agent_panel.rs

@@ -2135,6 +2135,7 @@ impl AgentPanel {
 
         v_flex()
             .size_full()
+            .bg(cx.theme().colors().panel_background)
             .when(recent_history.is_empty(), |this| {
                 let configuration_error_ref = &configuration_error;
                 this.child(