As noted in bug 605666, SVG images work just fine as the images in a "background theme" (formerly "persona"), but getpersonas.com and now addons.mozilla.org don't allow you to use SVG images as your background image. In particular: at the theme creator page... https://addons.mozilla.org/en-US/developers/theme/submit ...it says "PNG or JPG" underneath the file-pickers. Filing this bug on supporting SVG images there, as well.
(not sure if "Developer Pages" is the right component; it looked like the best fit, from a quick skim. Please reclassify if it's incorrect -- thanks!)