I've found a partial fix, by hacking the default .theme file using Windows Style Builder. (Unfortunately, it's not a free tool. And you need the beta version from the forums for Win8 support.)
- Launch Windows Style Builder
- Search for CONTENTALIGNMENT using the search box (there's only three of them).
- Change the entries from "Left" to "Center".
- Test, then save and apply the theme.
Once the .msstyles/.theme file is saved, you should be able to use it without having WSB installed on the system.
This is only a partial fix because it only appears to work on windows that use the Ribbon control (e.g., Explorer, Mspaint, Wordpad, etc.). It doesn't seem to work on normal applications/dialogs (e.g., Calculator, Charmap, etc.).
I've tried a few other things with WSB, but haven't been able to coax it into doing what I want. I'm not well-versed in theme editing, though, so maybe someone else will figure it out.