Dark mode visual problems

[bjones1]

Dark mode still has a number of visual rendering problems -- fixing this will probably require a serious deep dive in the CSS in runestone/accessibility/css. In particular, the text in a HTML class sidebar (generated by a .. sidebar:: directive) is unreadable. Text in a .. code-block directive (which translate to an HTML class code) is still in light mode.

[campbelle2]

Working on this