> Any CSS-suggestions for H1..H3?

FWIW

these are my styles now in Display Settings.

h1 { font-size: 1.3em; color: #800000; }
h2 { font-size: 1.2em; color: #800000;}
h3 { font-size: 1.15em; }
h4 { font-size: 1em; }
h5 { font-size: .95em; }
h6 { font-size: .9em; }
[download]

Not perfect but a start (margins etc are missing¹).

Though good enough to easily read posts on my mobile.

Thanks to the dark-red color I can easily identify <h2> in posts like How (Not) To Ask A Question.

YMMV! :)

You are free to suggest improvements.

¹) well I'm not a designer ( yes, excellent sperm count :)