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.

assets/styles/ui/palettes.rs

@@ -75,6 +75,8 @@ impl Tokens {
             overlay: Color32::from_rgba_unmultiplied_const(255, 255, 255, 235),
             rubber_band: Color32::from_rgb(30, 120, 230),
             app_bar_text: Color32::from_rgb(230, 234, 242),
+            app_bar_hover: Color32::from_rgba_unmultiplied_const(255, 255, 255, 28),
+            app_bar_tab_edge: Color32::from_rgba_unmultiplied_const(255, 255, 255, 40),
             timeline_item: Color32::WHITE,
             timeline_item_error: Color32::from_rgb(250, 220, 220),
             timeline_marker: Color32::from_rgb(70, 76, 88),