Thank you for these reports!

The theme switching is an ongoing issue, and hopefully will be resolved this week. What's happening behind the scenes, is your browser is sending Kumos a message essentially saying "send me the light mode version of this site, override any other settings", and that forces it into the default mode. If you're browsing at night, I suspect you'd get the dark theme, regardless of your choice. I should have a fix this week that resolves this (and moves the theme selector to a page in preferences, with previews of the themes).

The header should also be resolved soon :D