Hello Miriam,
the premium version of the theme has included typography options where you can change the fonts very easily. You can read more about the premium version here: https://www.mhthemes.com/themes/mh/magazine/
Here you can see an overview of the available theme options: https://www.mhthemes.com/theme-options/
In the free version you will have to modify the code to change the fonts. If you want to use a system font, please add something like this to custom CSS:
body { font-family: arial; }
You can read more about that here: https://www.w3schools.com/cssref/pr_font_font-family.asp
If you want to use another Google Webfont you will have to modify header.php and then add the CSS. You can find instructions on the website of Google Webfonts: https://www.google.com/fonts
I’m not sure which subheaders you mean, but you can try this and see if that’s what you want:
.entry h1, .entry h2, .entry h3, .entry h4, .entry h5, .entry h6 { margin-bottom: 10px; }
Regards
Michael
