How to style Vivaldi's UI?

Similarly to Firefox, we can customize the UI elements of the Vivaldi browser too.

For the newest version, above 2.6:

  • Open vivaldi://experiments
  • Enable "Allow for using CSS modifications"
  • Open Appearance section in settings
  • Choose the folder you want to use
  • Place your CSS files inside it
  • Restart Vivaldi to see them in effect

To change the UI font, you can use this line in your CSS file:

* {
font-family: "your font name" !important;
}

Source: Vivaldi forum