Is there a way, to turn "menu bar" black? (Screenshot available)

I think it would look really sexy, I read and write in a black background.

I have managed to make page black and text white, but don’t know how to make this black.

Hello,

Try changing your system themes - This is Mint 18.3 OS:

What flavor “dark theme” you have used for Desktop? I know a couple of versions.

Frankly I don’t use this as to me it is not appealing at all. Only did it to demo. This is simply changing the Borders & Controls in the theme to a dark selection. My actual base theme is ‘Linux Mint’.