Have you added a closing </div> tag at the end of the section you wanted to style?

Yes, I have

Then please provide a link

https://the-sapling-encyclopedia.fandom.com/wiki/The_Sapling-Pedia

Well, the whole page is messed up, I don't even see the <div> you've talked about, and there are way too many column tags. There should be only one <mainpage-leftcolumn-start /> and one <mainpage-rightcolumn-end /> (left one should come first) with each of them having a matching <mainpage-endcolumn />

Fix that first and then we'll se what next.

I'm not entirely sure what you mean by "touching the other boxes", can you clarify?

I do see however that the layout breaks when the page isn't wide enough, it might be related to the order of the columns - the left column should come first.

You are right. it is supposed to come first, now the boxes have a little seperation, thanks!