Greasy Fork

I don´t understand how to put a background

Simply that, I don´t understand how to put a custome background image once you enable the correponding options

I've been away, so this is a bit late. But basically, once custom CSS is enabled, you want to paste "url(https://www.example.com/myBackground.jpg)" into the input field in the "Set a profile background" section near the bottom of the settings page. You need to upload your image to somewhere that gives you a direct link to the file.