I'm a big fan of Opera12 because of functionality and big personalization options. Vivaldi is looking to be very good replace for old O12... but there should be easy way to change that terrible default theme - inspired by Google and MS flat design. Colors is first problem, second problem is that, that every bar, icon, tab is really big and waste much of working space. I have notebook with 1366x768 - that's standard resolution still and in opera i have all bars mini as match as possible.
Your first post, so welcome!
All of that will come down the road. Remember this browser is still in the alpha/tech preview stage. But the whole design is geared toward maximum customizability.
I have a larger screen but still hate wasted space, and prefer other colours.
Here is a theme which may answer most of your wishes, as long as you don't mind having all of the toolbars at the top, and also you work with a maximized window :
Scroll down for the 2nd screenshot and the required CSS code.