I found a tool called “WinaeroThemeSwitcher” for Win7 and 8.
TS switches to your various themes without a GUI getting inbetween. Works from the cmd line, if you point it at a theme in one of the known paths.
Or you can set up a shortcut with a target similar to this:
^^ I have a few such on the desktop. They work nicely, because like I said, there’s no Personalization window coming up first.
Because of Aero Theme Switcher, I can now dump a bunch of rickety unreliable autohotkey scripts I’d been using for this job.
The only thing I’d call a drawback, is minor. For the current theme, there will be an “Unsaved theme” created as well. It’s probably needed, for reasons that I don’t know.
ETA: This program also works on Windows 10. I tested on the Fall Creators version. So far, no downsides. If anyone else wants to see if there are any bugs, the program is linked above.
Hope you like.