If you mean on the lines of this
Then yes it's possible, but there's a but. It isn't your Reaper theme that you tweak, but your OS.
In Windows you'll need to make a new .theme file, which is found in AppData/Local/Microsoft/Windows/Themes on Windows 7.
As you can see in the pic, I haven't cracked all of it, there's a tickbox I can't find the values for.
Steve