I am not willing to add a new theme element to the 13 that already exist, let alone define it in config-extensions.def rather than in config-hightlight.def. It is too much additional complication for too little impact. Also, any theme specified in config-extensions would likely not work if the user switches between dark and light text. Theme sets work together.
I will consider letting users select which of the existing 13 to use. This requires that users be able to know what the 13 choices are. Hence the dependence on #22705.
I might also look at the visual effect of underlining.