Solved thanks to @MrGrey in the comments! Leaving the post text here for posterity.

Whenever I join a new site one of the first things I do is customize the style to my liking with CSS skins. I'm having a little trouble here, though, specifically with the fact that no matter what I do, I can't increase the width of the paragraphs in posts and comments.

As far as I can tell, the div.topic-text-full element spans all the way across the main element, with just a little margin space. .topic-text-full's child p elements only span maybe 80% of .topic-text-full's width, though, and my attempts to make them wider have met with failure:

.topic-full-text p {
  width: 10%;
  }

successfully shrinks the element, and so do any other widths under 100%. Setting it to any number above 100% makes no changes. Similarly, setting the width in pixels can shrink the element width, but not increase it.

The same goes if you replace .topic-full-text with .comment-text.

Is there something very simple that I'm missing? It just really bugs me that the paragraphs don't go all the way across, but since that's such a small issue I'd rather fix it on my end rather than file an official complaint about it.

Now, this is in solarised dark (1 true colourscheme represent), but this specific issue is present in all current schemes - the blue and purple are way too close together. This also impacts visited vs unvisited links.

On this same note, the button style for solarised dark is not colour-shifted between schemes.

I had a look at the available themes and being a person who is sometimes inclined to rice, decided to port my colourscheme to tildes using the Stylish firefox extension. Unfortunately,
Content Security Policy: The page's settings blocked the loading of a resource at self ("style-src")

comes up in the debugger on every page. How can I work around this?

There were a few things that I was a bit uncomfortable with, so I created a few css styles to improve some aspects of the experience.

Reddit like theme
I'm used to Reddit, so I created a quick theme to make Tildes a bit more like Reddit: https://pastebin.com/1rMhbFMF

Hidden tags
Seeing a [Joke] tag can sometimes ruin a joke, so I created a snippet that hides tags unless you hover over them: https://pastebin.com/Bzvr3Vmp

Moved the submit a comment box
I made a snippet to move the comment box to the top of the page: https://pastebin.com/WCqv3Mr2

When posting a comment, the width of the text seems limited. When I removed the max-width from the p and li element, it filled the box as I expected it to. (Source)

To me, the second one looks better. Some padding left and right could be added, but I certainly wouldn't use a fixed maximum width there (A percentage would be fine, I guess).

Edit The seems seems to apply for posts as well.

When I upvote a post, it just goes form dotted border to solid border and a very slight change in color which makes recognizing if you already upvoted a post a little hard.

Maybe inverting the color of the box would be more suitable?