You probably landed here from Google searching for something like “change menubar color in macos monterey” or “make menubar non-transparent”. This was the first thing I myself started searching after installing the newest version of macOS, because the Monterey Gap yawning at the top of the screen is just purely annoying, especially in the dark theme. It is just purely out of the style of the rest of OS.
Thankfully macOS contains built-in tools to get rid of it.
System Preferences ➡ Accessibility ➡ Display ➡ Reduce transparency
Note: it will change the color hint slightly, make windows colors less “hip”, but The Monterey Gap will be gone.