CSS is pretty simple but modern sites have hundreds to thousands selectors. If you want to do anything worthwhile you need to modify big part of those. And hope new updates won't change anything making your custom layout a total mess at some random point.
I tried to theme some popular sites to my taste of dark themes and it's really tedious.
Obviously every site is different, but I've developed a few styles before for sites like DDG and wikipedia, and it's a job measured in hours, generally. Nowhere near the amount of work involved in most OSS, and plenty of people here work on OSS.
