I see options to change the color of lots of other things but I don’t see where to change the background color of the menus, I can only see where to change the text font.
The only way I am aware of to change the menu background color is to change the theme (Window / Themes).
Search for Theme in this Forum to discover how to create a Theme for Scrivener.
Essentially, rename and unzip the .scrtheme file saved with Save Theme to file, and change the Palette, Stylesheet and Preferences files. ZIP and rename back to .scrtheme and Load Theme from File. Restart Windows to see your new Theme.