In the "for what it's worth" catagory:
I gave up and restored the previous (4/21/08) MyColors version of Diamond I had backed up prior to installing Impulse.
I had tried uninstalling via Impulse and manually deleting the related theme folders (WB, Icons - Impulse only deleted the .mycolors file) as well as doing an Explorer search on Diamond*.* and deleting all of the related files that turned up, then re-installing via Impulse, still no go. Tried deleting through MyColors and manually deleting folders and files and re-installing via Impulse, still no go. Deleted again, tried to install via MyColors, couldn't get a download.
Said the heck with it, "installed" again via Impulse (or else Impulse won't know it's installed), but didn't "run" (load or extract) it. Manually deleted the newly downloaded .mycolors file from the Themes folder, copied my backed up previous .mycolors file over to there. "Ran" it via Impulse, worked like a charm. Impulse and MyColors don't know the difference, don't know there's a newer version out there.
Maybe support could get you the previous MyColors theme version?
WWW LinkOn XPHomeSP3, current Win/Impulse/MyColors/WB/etc. Just did all this tonight.