I wrote a uBlock rule to fix the new Wikipedia theme.
To me it felt like the text was kinda floating in the void, not anchored to anything (especially once you're scrolled down the page), so I added a vertical rule between the sidebar and the content:
[wikipedia.org##.mw-table-of-contents-container:style(border-right](wikipedia.org##.mw-table-of-contents-container:style(border-right): 1px solid; border-image: linear-gradient(to bottom, white 0%, #a2a9b1 10em) 1;)