UPDATE: After some research, it was determined custom theme support was not blocked in any way. The inability to apply themes appears to be a bug with the resource cache tool, patched by KB933928. The root of the problem, however, lies with theme authors. Themes MUST have a properly (matching the .msstyle name) named MUI otherwise Bad Things Happen™.
Improper layout:
C:\Windows\Resources\Themes │ Vista-Classic.theme │ └───Vista Classic │ Vista Classic.msstyles │ ├───en-US │ aero.msstyles.mui << wrong! │ └───Shell └───NormalColor │ shellstyle.dll │ └───en-US shellstyle.dll.mui
Proper layout:
C:\Windows\Resources\Themes │ Vista-Classic.theme │ └───Vista Classic │ Vista Classic.msstyles │ ├───en-US │ Vista Classic.msstyles.mui << right. │ └───Shell └───NormalColor │ shellstyle.dll │ └───en-US shellstyle.dll.mui
Anti-tgtsoft