Hello @ondrej
thanks for the details.
I am afraid I couldn’t reproduce any of your reports 
I tried all of your mentioned things and everything looks good for me.
Be it the post badges theme component
Be it the versatile banner
Or the Light <> Dark switch
Especially your checkbox for Light and Dark gives me the one and only conclusion:
you are using an outdated Browser (e.g. Internet Explorer 10), which I (won’t) don’t support. Is this right? 