Turn on Preferences / Toolbars / Appearance / Use Office 2003-style for toolbars and you get more control over the menu colors as everything will then be custom-drawn instead of using your Windows visual style.
I had that set originally, but then I was having problems finding how to change the background of the menu from solid white. The sidebar of the menu would change, but I couldn't get the menu background itself to change.
Oh, that's true. 
From your Firefox/Opus screenshot, it looks like Firefox is using a completely different visual style to draw its menus. Have you installed some kind of custom theme for Firefox or something? Or are you using something like WindowBlinds set to use different visual styles with different programs?