How Make Sage CSS "Portable?"

I use Portable Firefox Portable (we're all in transition, here, aren't we?) and the SAGE extension for my RSS reader. How can I make Sage's variable CSS reference "portable?" My CSS files are stored on my USB flash memory drive. The CSS option in Sage nails the chosen CSS file to the drive letter. Of course, the drive letter changes as the flash drive is moved from PC to PC, so the CSS reference is lost (incorrect) and SAGE reverts to its internal default "CSS." [By the way, I use PSTART as the portable launcher in case there's a variable path PSTART sets that would be useful here in solving this problem.] Thanks, oh wise ones!