I want to change the background colour for the main menu and the top menu to a shade of purple.

There have been other threads which talk about changing the css in main.css and also the background css is the css/themes folder.

Problem is I can't fathom what in those css files to change.

For instance the background colour for the blue theme is #0077bb just looking at it in an image editor.

However there is nowhere that this colour occurs in the css files that I can find.

Where do I look?