Here is the userChrome.css that I use with Firefox to match my I3wm theme. I also modified its structure because I don't support tabs above the touchbar.

Not being a developer, I associated pieces of code that I found:

• on different internet sources for the colors.

• on the reddit group r/firefoxcss for the hidden bar part.

To use this file on Windows:

1. Download the userChrome.css file
2. Go to the folder C:\Users"username"\AppData\Roaming\Mozilla\Firefox\Profiles"profile_mozilla".default
3. Create the folder chrome if it does not exist
4. Copy the file userChrome.css
5. Open Firefox and type about:config in the address bar
6. Look for the line toolkit.legacyUserProfileCustomizations.stylesheets and change its value to true
7. Restart Firefox and enjoy

To use this file on Linux:

1. Download the userChrome.css file
2. Go to the ~/.mozilla/firefox/"profile_mozilla".default/ folder
3. Create the chrome folder if it does not exist
4. Copy the userChrome.css file
5. Open Firefox and type about:config in the address bar
6. Look for the toolkit.legacyUserProfileCustomizations.stylesheets line and change its value to true
7. Restart Firefox and enjoy