However, the "#EXPAND-PLUSMINUS" is not prefixed with any class. So what happens is they will try overriding each others. Many times we have to specify "!important" to reinforce particular css rules. Obviously this work-around will cause other maintenability issue.
It will also cause flickering issue in designer studio, if you changed line-height or font size. To be more specific, when you mouse over whichever section with a header, the font size/line-height of entire section(not only the header, maybe due to inheritance) will be changed according to "#EXPAND-PLUSMINUS:hover" snippet. As consequence, the overall height will be extended or shrinked accordingly, Which make it extremely hard to click on one particular control. You can imagine how it can decrease the productivity drastically.
This sounds like a product bug that would potentially require a hotfix to resolve the issue. If you would like to get this fixed could you create a Support Request (SR) and reference this post in SR? Once you get the SR number, could you add the number as a comment to this post so that the moderation team may track it?