Title bar: outline inactive tabs and keep the Home tab readable on hover (#1116)

Inactive document tabs get a faint outline so neighbouring tabs stay apart, and the Home tab uses the same hover tint as the design tabs so its label stays readable in the light theme. Fixes #1115.

crates/ui-egui/src/theme.rs

@@ -122,6 +122,10 @@ pub struct Tokens {
     pub rubber_band: Color32,
     /// Light text on the application bar.
     pub app_bar_text: Color32,
+    /// A hovered tab on the application bar (keeps `app_bar_text` readable).
+    pub app_bar_hover: Color32,
+    /// Outline of the inactive tabs on the application bar, so neighbouring tabs stay apart.
+    pub app_bar_tab_edge: Color32,
     /// Timeline items: normal and failed.
     pub timeline_item: Color32,
     pub timeline_item_error: Color32,