Vivaldi browser 1.3 stable ships with the brand new options to create and customize vivaldi themes directly in the browser.
Create new Vivaldi UI theme in Vivaldi Browser:
Update your vivaldi browser to the latest version, that is vivaldi 1.3 and later.
Click the vivaldi menu and go to Tools->Settings. It will open the settings window. Then select the Themes option in the settings window.
Then click the plus button from bottom of all available themes. Then change the background, Foreground, Highlight and Accent colours. And give a theme name and hit the save button. That’s it.