To add to this, I think a simple theme builder like Slack would be really nice.
Ideally it would also allow for basic changes like Condensed/Regular (line height), Max Width of editor (like @diibv mentioned), Header size difference (make each header x times larger than the next, and let the user slide to change X) with some preset themes for what you generally work on "journaling", "code notes / math", "night mode" etc.
As Laurent explained to me,
I am happy to create mockups for how this would work if anyone wants to collaborate on building it into a plugin.