Currently, both .ext-discussiontools-visualenhancements-enabled .ext-discussiontools-init-section and .ext-discussiontools-visualenhancements-enabled .ext-discussiontools-init-section > h2 selectors have !important in their font-family definitions. This makes it harder to customise fonts, since you also need an !important in any customisation to make it work, which doesn’t make much sense. I prefer default serif font headings and wanted to customise discussion header for myself to this:

but to make it work, I had to add !importants everywhere, even though CSS handles specificity fine anyway on those selectors. It is a small issue, but I suggest you’d reconsider this, since usually !importants are discouraged without a good reason.