List of steps to reproduce (step by step, including full links if applicable):
What happens?:
The 'cancel' button is completly red.
What should have happened instead?:
'cancel' should be readable. I'm not sure, but I think it was white letters on red background.
Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
MediaWiki 1.37.0-wmf.16
The combination of the quiet and destructive flags for the old non-OOUI styles (WikimediaUI base, I think? Maybe @Volker_E can correct me) is working out poorly. Destructive is a button with a red background. Quiet+destructive should be red plain text, without the button background, but for some reason it does not get removed, so you get a button with red text and a red background. As far as I can tell, the issue is not specific to Flow in any way (none of the CSS rules involved are from the Flow codebase).
In case you don't have delete rights for some Flow page, the relevant HTML is
<a class="mw-ui-button mw-ui-quiet mw-ui-destructive" href="" title="Cancel" data-flow-interactive-handler="cancelForm">Cancel</a>
The problem seems to be some sort of specificity mismatch between the transparent-background rule for the quiet class and the red background for the destructive class.