Vivaldi, like some other apps can't be skinned hence it uses it's own skinning engine.
I don't like when programms use their own skinning engine, the make me very angry! Because I preffer a consistent system, every app with the same skin, borders, colors and fonts. I also use resource hacker to change icons, fonts and colors, sometimes even hex editor to achieve that. And I hate borderless windows. Therefore for example, I am still using office 2010. When there is an app which I cant skin with the mentioned methods, I simply dont' use it on my customized and skinned system Well...but there are exceptions. I need a browser....
And I made a screenshot from the skinned chrome, a screenhsot with with the borders and placed it in the common.css as background. Now my Vivaldi looks like it is skinned