Customizing editor experience

Hello everyone, I’m working on a e-book maker app where the main feature is a rich text editor inside which authors can write book drafts. I would like to customize the look of the editing experience. Right now, I’m not applying any custom styling to the nodes (h1, h2, …, ul, ol, code, pre, etc.) inside the editor (see screenshot to see what is currently rendered). In order to do that, I would need to pick a custom font, custom spacing between headers, etc. I’m not very experienced with the frontend, so I was wondering how you all would approach this. Where would you go for inspiration? Are there any free typography kits available online? Our project has a global style sheet and I’d like to stick to vanilla CSS for implementing the custom styles.