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By clicking on "more channels ..." and then in the dropdown menu, the items appear to be white, the same color as the background, and you can not see the options.

A temporary workaround for this is the following:
Administration --> Layout --> Custom CSS, paste the following
aside select.c-select option { background-color: #04436a; }
Works perfect, @Jannibal. Thanks.
Now a new problem:
After removing this custom CSS, it still continues to influence the layout.

Even after removing the CSS and reloading the page, the background still looks as if the CSS was applied.
@rauhmaru Can you open a new issue for that?
Roger that :)
Issue #5725