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