Both of those settings are set to "Default (D)", i.e. they are both disabled.
This behaviour is totally random, but once that a page/site is affected, there is no way out other than restarting Opera.
I took a video, the background is supposed to be transparent, but unless it's explicitly set to a color, it sticks to black:
It seems that, at times, the browser default style for background-color:
defaults to black automatically.