Firefox's semi-transparent UI on macOS works perfectly in both light and dark modes.

Light theme

Dark theme

Work with Firefox Color extension

1. open your profile folder by type about:support in the address bar, find the "Profile Folder" and open it.
2. Download the source code, copy the user.js file and chrome folder into your Firefox profile directory.
3. Restart the browser.

• Only tested on mac, doesn't sure what will happen on Windows.