I was also wondering if theme chooser works. Can you try it? In settings there should be a way to "Switch theme". If there isn't, then it doesn't have theme chooser. CM, AOKP, PACMan, and some other AOSP roms have it. Anything based off of CM *should* have it.
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I looked and Slim doesn't have theme manager. Their stance is that "the theme manager sucks", so they are building their own setup for themes. IIllusionRom doesn't specify if they are *pure* AOSP (building from source and then adding their own stuff) or pulling from another rom (PA, CM, AOKP, etc). They do mention HALO from PA, so they may be building off of PA. I'm not sure if PA uses the TMobile theme interface or not. I'd be willing to help port CM or something of the sort, if you need help gigilie =]