As much as I like Lemmy, I was growing tired of the fixed width styles and decides to write some quick User CSS overrides to make it more compact and use the entire screen width.
Here's a github gist with the styles -- I haven't tested everything so there could be bugs. You can use a browser extension like Stylus for Firefox to apply the theme. I have tested with darkly theme only so far, but I imagine it'll work for litely as well.
I made something like this a while ago but didn't release it because I forgot to I guess (and it sorta sucks)