Header Text No Longer Cyan in Reader Mode after Obsidian 0.15.6

[marchank0]

@harmtemolder
I added a new selector to fix color for headers in a reading mode

pls let me know if it should be updated: I'm not a css guy

[harmtemolder]

It looks like the Obsidian have introduced a new .markdown-rendered class around all content, so it's cleaner to use that than to circumvent it using some other selector and !important. I've started on a new version of this theme that takes that into account and will solve #23, among other issues.

Thanks for the pull request though, I really appreciate your help!