Same theme that works on two PCs won't work on a third one

[Ascaris]

I have a theme that is just perfect for me in Windows 8.1.  It's one that is a heavily modified version of a Win 7 theme from when I used 7... now I am on 8.1, and I long ago used WSB to port the theme over, which was successful. 

Now that theme works flawlessly on two of my PCs running 8.1.  I just put 8.1 on a third (I don't like Windows 10), but the very same theme that is flawless on the other PCs does not work (I've tried the unsigned theme service and a patcher for the three files, with the same results). 

The .theme file works (it sets the wallpaper and theme color), but the .msstyles is not loading.  It merely uses whatever the last one loaded was.

WSB (newest beta 1.5.6.3) on the two PCs where the theme works can test the theme just fine, and WSB does correctly apply the custom theme when I hit the Test button.  When I try that on the PC where it is not working with my theme, I get an error message:

"Error testing MsStyles File

There was an error applying the Test MsStyles File, this usually signals an error in the MsStyles file."

If there's an error, why does the same file work on two other PCs?  There must be one in there somewhere, but where?

I've tried a custom theme from the web for 8.1 on the PC that isn't working with my theme, and it works fine!  So apparently there is some issue with my theme that prevents it from working only on one of my three PCs, and it also prevents WSB from applying the theme in a test.

Any ideas what I can do to narrow down what this error is?  Thanks!