If the default "Primary Styling CSS" or the user's currently selected "Syntax Highlighting CSS" changes in a release, and the user hasn't modified the previous default, then the user's styles should update to the new default.

Alternatively, the user could be prompted. Perhaps they prefer the previous default -- even though the new one should be better (by some measure) or it wouldn't have been changed.

I guess the ideal would be to save the previously set CSS as an easily selectable value, in case the user wants to revert, and set the new default as the current. (An even-more-fanciful ideal might be to diff/merge the new and the user's current, and only replace those rules that haven't been modified. That's getting pretty tricky/dangerous, though.)

For new users, this could be mitigated by not immediately writing the default as the user's setting. Then it would only get set if they modified it.

This problem is especially heinous right now because a default CSS change in 2.7.0 was needed for the new Markdown table column alignment to work. But users have to manually reset their CSS or else it appears broken. For example: #47