I’m having the exact same problem as @Jonas had and I tried all suggestions of both @sunny and @StrangeTech but the styles I defined in the post template don’t apply in the post editor and I can’t seem to figure out what else I can try to fix this.
On the front end (live site) everything works perfectly, but in the back end (post editor) the styles from the post template don’t seem to carry over. I checked the DOM in the post editor and the .blog-post-content global class (where I defined relative styling for H1, H2, etc.) I attached to the post content block in the post template is not being applied in the post editor, so I understand that this is reason for my situation.
I tried defining the styles in the post template through global classes with relative styling, custom css as well as a custom stylesheet. The only styles that carry over are the ones I specify in the custom stylesheet and only if I use the .is-root-container.wp-block-post-content selector as proposed in the forum post Making Posts editor client friendly.
Changing the visibility conditions on the post template as you (@StrangeTech) suggested unfortunately does not change anything.
Does anyone have a clue what other options I could explore to try and solve this this issue?
Thanks a lot for taking the time!