fix invisible side boxes (closer summaries) under dark mode

[aaronliu0130]

Please ignore the "random bold text" in my screenshots; that's an extension called Bionic Reading.

[jwbth]

.side-box is an enwiki-specific class. In Comment.layers.less, we have only MediaWiki-wide stuff. If tweaking the styles for enwiki, then we need to put it in https://github.com/jwbth/convenient-discussions/blob/main/config/w-en.js.

But do you have any ideas how to fix this without resorting to tweaking local styles? (If not, that's completely OK – I have a comprehensive knowledge of the script and will try figure it out myself.)