Hm, sorry for being slow, but I've generally stayed as far as humanly possible from things like gui profiles, so I still don't really know that well how they work... I found the guiPlatformGenericMenuBar.ed.cs and .gui files in tools/gui though. They're not very big and the changes seem to all be occurring in the profiles, but I can't find actually find a definition for the referenced "GuiMenuBarProfile" anywhere in the project, or even the more generic "MenuBarProfile" which is referenced in several other guis.
I tried changing the reference for PlatformGenericMenuBar to a new profile called "GuiDarkMenuBarProfile", and then making a profile of that name, by copying one of the other profiles in profiles.ed.cs, using the DarkUI values for fillcolor, textcolor, etc - but nothing changed. I'm not sure what specifically menu-related stuff the actual GuiMenuBarProfile is supposed to have.