Currently Gateway does not patch sig checking of system titles. Themes qualify as such, so any custom ones will not work for now. Since themes didn't get introduced until. 9.x firmware, currently available public CFW can't make use of theme CIAs because they can't boot the required firmware that uses it.
You can install CIAs of retail themes that aren't modified. That's about it. Custom stuff however, you will be relegated to the extdata tools like CHMM or the extdata tool that preceded it.