Hi @Loki, Right. Its easy peasy!
Solution 1: You can directly mention the css code in the Rasa Webchat snippet which is every easy, please see this tutorial video: Best Ways To Customize Rasa-webchat - YouTube
Solution 2: If you have a project folder (clone from the Rasa Webchat) and running on npm server node 3000 I guess for ReactJS app; then the process is all different.
But, If you only using the Rasa Webchat Snippet code and want to customise using your own CSS, then please follow my solution thread: Integrating Chatbot-Widget ((https://github.com/JiteshGaikwad/Chatbot-Widget) ) to a Wordpress site? - #7 by nik202
Please ask me endless question on this!