This is the Swiss theme I made for the upcoming football world cup. It features a red and white theme with a dark panel as contrast. The startpage background is a picture of the Matterhorn, one of the most distinct landmarks of Switzerland. Translucent speed dials allow for good visibility. As a small extra the panel has the Swiss national flag added to the top. It’s not completely without actual purpose, because you can drag and drop the browser window by clicking on it, which is normally hard to do on the upper left side of a Vivaldi browser window.

Startpage

Cropped and saved for use as startpage background. Right click image to download.
Source
Speed Dials

Settings

Background: #ffffff | Foreground: #333333 | Highlight: #d52b1e | Accent: #d52b1e

Custom modifications

For this theme I modified the Vivaldi UI and you can do the same. Save the linked code as custom.css file and move it into the Vivaldi application.
Instructions
CSS Code