The "Default" theme, as featured in my video about Workspaces and Custom Icons. Download other themes from the series here.
You can watch the video on YouTube. Read more on how you can play with Workspaces and Custom Icons in Vivaldi on our blog.
Image by Freepik. Featuring Uicons by Flaticon. The icons were modified and adjusted to fit Vivaldi's UI. Feel free to grab them and make your own theme with this set.