How to specify a CSS file to one particular .region file?

I have a few .region files, with a corresponding .less file for each. I'm starting to get duplicate elements, such as <h1> tags in more than one .region file. I would like my .region file to refer to one and only one .less file. It appears when I add a styling element to one <h1> tag in a .less file, it applies to all of my pages. Thanks!

I'll use the built-in editor to add CSS for now unless there's a workaround to being able to use files