https://userstyles.world/style/10168/1440p-lemmy

Here’s an extension to add custom styles to websites:

https://add0n.com/stylus.html

And this is the custom style i’ve wrote:

.container-lg{
    max-width: initial;
}

.my-3{
    margin-bottom: 0px !important;
    margin-top: 0px !important;
}

No, I don’t see anything in the settings either.

I wonder if a simple GreaseMonkey script can work to apply your own CSS

FYI kbin.social has a nicer UI than Lemmy, if you ask me. Still federates with any Lemmy instance and even connects with Mastodon too if you’re into that. Otherwise yeah, only way to mess with the UI here is with browser hacks unfortunately. I agree it’s way too squished.