So you are opinionated and think you can do CSS better than me? You probably can. So let's help you do that.
Making themes is easy. A theme is just text content. And so we can post themes to the site just by making a text post. The best place to add it is /s/Themes.
It would be 99% CSS. But there are a few extra tricks I should tell you about.
First is starting it with three backticks. This will be ignored by the CCSS parser (custom css). But it will make the post content more legible for anyone else by putting your content into a code block. Also then you should end your post in three backticks.
Next is the #import statement. It will add the content of any other theme to your theme. Just use #import /the/relative/url, or #import https://the.absolute/url, or #import postid. You could even mix and match themes with only import statements.
This way of doing themes also means anyone can easily read the content, and make small modifications to a theme.
Happy theme building!!