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.