Even if your theme doesn’t use Sass, you could just rename your .css file to .scss and add that compressed property that @mmistakes mentioned. You don’t have to actually use any Sass techniques in your CSS.
When your site builds, Jekyll will minify the CSS. It’s a great approach. In my opinion, Jekyll’s docs on Sass could be improved a bit, but the info is there. https://jekyllrb.com/docs/assets/.