I'm not a designer, but one possible improvement would be to cap the width of the text regardless of browser width.

While I agree, to a certain point (depends on the width), not everyone likes this.

Which is why the styles should be something users can adjust when logged in.

It's very weird that the only real customizable part of this site is the background color of the topbar, and that's a perk that requires karma.

You can do this already with Chrome extensions on desktop.

I like the 'Stylish' chrome extension. User created (or custom) styles for any website you use regularly. I use dark themes for practically everything.

My Hacker News style is a user stylesheet "Hacker News Dark" and 1 line of additional CSS I added to reduce the lane width of the text:

.comment-tree { max-width: 600px; }

Of course you can go to town and add comic sans, etc.

That's precisely what I did with Stylish, I pickup a nice dark theme and the UX is awesome now