theme: Fallback to opaque color for title_bar.inactive_background (#16833)

Fixes https://github.com/zed-industries/zed/issues/16699, fixes
https://github.com/zed-industries/zed/issues/15112, fixes
https://github.com/zed-industries/zed/issues/14955

| Before | After |
|--------|--------|
|
![image](https://github.com/user-attachments/assets/3c93dc02-3421-4fd8-b34e-c54644202caa)
|
![image](https://github.com/user-attachments/assets/f938d77f-7e9b-4c1f-9beb-38ff77a5fa93)
|

Release Notes:

- Linux: Fixed title bar becoming transparent when the window lost
focus.

crates/theme/src/default_colors.rs

@@ -50,7 +50,7 @@ impl ThemeColors {
             icon_accent: blue().light().step_11(),
             status_bar_background: neutral().light().step_2(),
             title_bar_background: neutral().light().step_2(),
-            title_bar_inactive_background: neutral().light_alpha().step_3(),
+            title_bar_inactive_background: neutral().light().step_3(),
             toolbar_background: neutral().light().step_1(),
             tab_bar_background: neutral().light().step_2(),
             tab_inactive_background: neutral().light().step_2(),
@@ -148,7 +148,7 @@ impl ThemeColors {
             icon_accent: blue().dark().step_11(),
             status_bar_background: neutral().dark().step_2(),
             title_bar_background: neutral().dark().step_2(),
-            title_bar_inactive_background: neutral().dark_alpha().step_3(),
+            title_bar_inactive_background: neutral().dark().step_3(),
             toolbar_background: neutral().dark().step_1(),
             tab_bar_background: neutral().dark().step_2(),
             tab_inactive_background: neutral().dark().step_2(),