AFAIK most of the Vivaldi GUI background color are auto adjusted. Now, from the screenshot you provided I found your color palette somewhat problematic - both Background & Accent colors are set to #000000! This is a formula for terrible background contrast. The purpose of Accent color is to provide secondary highlight to Highlight color, so you should set it to be slightly lighter or darker than BG color, but never exactly the same.
I suggest you set the Accent color to something lighter, like #333333 , warmer color #443333, or cooler color #333344. Basically, in general there should be at least 15% of contrast between 2 colors for your eyes to see a distinctive differences from a screen in close range, & 30% contrast from far away.
So, even if you setup a Dark Theme, there should be some distinctive level of depth between elements to help your eyes recognize 'em easily. Else, your eyesight will be harmed overtime, cuz you are forcing your eyes constantly focusing extra hard to recognize elements hidden in the dark from a screen. 😉