Well, I don’t even know where to start! There are dozens of .css files floating around ERPNext. The solution that the MN Technique team had recommended to me is to put a style block at the bottom of the HTML template (like this).
I guess I’m looking for a recommended path for creating ERPNext-style forms in a custom app. And maybe a brief overview of the CSS approach in ERPNext?
Thanks @makarand_b