Redactor Editor With Dark Background - 5.7.3.1

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I have some content blocks with dark backgrounds and white text. The dark background is selected using a custom css class on the block, thereby surrounding the content block with a div.custom-class.

When I edit them I get Redactor with a white background and white text... not helpful.

I have put some css in place to make the text black when editing, but I'd like to be able to show the block in edit mode in the same way it'll look when live. Any ideas?

Thanks