I agree it's strange that there are any hardcoded styles in DNN. But on the other hand if we want to change something we can and if you want to contribute to the project feel free to do so.
By the way this has nothing to do with light text on dark backgrounds because if you wanted to make the background yellow, the problem would be the same.

What I don't understand is that you write "in some browsers"..... Are there browsers where you can change the background-color?

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