If anyone knows how I can add a custom background image too, I'll deeply appreciate it :D

Hey, this is a beautiful theme, without background image too! But if you want to add an image, just press the Background button in the editor (starting with Vivaldi 5).

Thank you very much! Yeah I'll probably leave this as it is and make another version with a background :D

Great theme! It really brings the atmosphere and feel of it everywhere, even on webpages, and that's pretty hard to do!