Caption Background

Hi,
There is a thick black bar at the bottom of my images. That’s where the captions appear. Perhaps it is the background of the whole thing.
Anyway, it is black. So my black captions cannot be seen in there! I change the captions to white but sometimes they mysteriously go back to black.
My question is, how to change that captions background to white? So black captions will appear without a problem?
Thanks!

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