MathJax
As of DSpace 7.4, this is now supported via a configuration option and doesn't require theme changes. I've changed the configuration and verified that HTML is displaying in the abstract of the noted item. According to the documentation, MathJax is also supported. This is finished, I think, but I'm trying to find some examples of abstracts with MathJax.
Keith G - I tested with a MathJax item; each snippet of MathJax was displayed twice. Example item: https://vtechworks-dev-ds7.cloud.lib.vt.edu/items/ff3f0481-97f5-4ba6-abe9-7c2ee8f963f7. This is a known DSpace bug; a fix may be available soon.