[OcSelect] Background is not completely invisible

[dschmidt]

At least in dark mode the background of the OcSelect component does not seem to be completely invisible.
When not expanded, you see it on the top corners but also the bottom corners.
If you expand it, it's fixed for the bottom corners.

Steps to reproduce

Look at settings (e.g. https://host.docker.internal:9200/settings)

Screenshots (if appropriate):

Browser log

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a) The javascript console log
b) The network log 
c) ...