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