I want to make the headings the same size (H1-H6) - and this can be done in the settings.

But I also want to make the headings different colors and backgrounds, so that the H1-H2-H3 different.

Is there any way I can change the CSS files?

Hi,

There’s not currently a way to apply custom CSS to the editor our output preview, so you’re not missing an existing feature

It's a pity, I'll try to unpack app.asar and try to do it myself.