Webflow follows a common, clear paradigm- design is the domain of designers, content is the role of content-creators. The editor is designed for content creators, so it does not permit any mucking around with the designer’s CSS.
That said, there are a handful of situations where this might make sense in specific, controlled situation, such as inside of a blog article or press release where you may want to break all the design rules with a big red notice or some such thing.
The tool best suited for that is Refokus Rich Text enhancer, which allows you to insert a kind of CSS markup in your rich text content.
If you’re looking for more complex control over a chunk of content, there’s always the HTML Embed sub-element in rich text elements, where you can create whatever you want.