UserContent.css file with google search page in a dark theme and the firefox start/new tab page also in dark theme.
- Open Firefox and press Alt to show the top menu, then click on "Help → Troubleshooting Information"
- Click the "Show Folder" button beside the "Profile Folder" entry
- Create a folder named chrome in the directory that opens
- In the chrome folder paste the .css file with the name userContent.css