Custom CSS for communities

Is this a possibility? Right now, lemmy communities are very compressed, fitting into the centre third of my screen. Smaller devices, such as 1080p devices, will fair better - but I'd ideally like to implement some CSS that expands page content a little, perhaps to 70vw or so. I could do that with Stylish or some other scripting addon, but I'd like to make it a default for visitors. Possible, or no?

Here's what it looks like currently: