I don’t know how you got maroon styles by default without doing it deliberately, or perhaps you started from a project template where the designer of it made this choice, and saved their stylesheet into the template?
Whatever the case, if you create a new project from Blank, or any of our built-in templates, you should be seeing simple black styles.
If you want something a bit simpler though, you might be interested in this setup. As you can see, it was designed for those that primarily use Scrivener to draft plain-text, like Markdown and LaTeX, but it’s just some aesthetic settings, so anyone can use them. You can probably ignore the .prefs file, and just take a look at the “plain-text-emulation.scriv” project.
First, check out the default stylesheet to see if that’s more aligned with your aesthetic (Format ▸ Style ▸ Styles Panel). As I recall, none of the styles actually change the font size. If that looks good to you, or at least closer to what you want as a starting point, follow the instructions provided in the “Information” section it loads into.
As hinted at above, you can make a custom stylesheet your starting point via the project templates feature. Modify any of the existing templates how you please, and then use File ▸ Save as Template..., making sure to tick the option at the bottom to save the styles into the template.
How do I change that? I cannot do it through Format > Style route it doesn’t let me change anything. The Project > Settings way only is for the body text…
Maybe take a look at §17.3.3, in the user manual, on Redefining a Style. It would be insane of us to create a style system that you couldn’t modify.
I know styles are meant solely for compiling, but I want my chapter headings to be different from the body font.
I wouldn’t say that! Styles are an extremely valuable tool while writing, as they let you find text marked for a particular purpose, among other things. That they can be changed by the compiler is in part what allows them to function in this way. You can use styles that, in the end, aren’t even visible in the output and just look like normal text.