For granular configurations like this, "flags" mght be the place to look. Unfortunately, your request isn't there.
However, you can check it out anyway, to see what's there, if you're unaware, enter this into the address bar:
chrome://flags
it'll prob result in:
opera://flags
Either way, you're in the "Experimental" settings.
There's lot's of them - you can search in the provided search box, such as "dark" or "bar", etc