List of steps to reproduce (step by step, including full links if applicable):
Go to https://en.wikipedia.org/w/index.php?title=Special:ExternalGuidance&from=es&to=en&page=ElsaBornFree&targettitle=&service=Google (can reproduce with/without safemode, and in both new/old Vector and Timeless)
The second button, directing the user to edit the article in the source language, is illegible because the background and foreground colors are identical (blue text on a blue background)
What should have happened instead?:
The button should have blue text on a white background, per the appearance in the UI example.
Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
Firefox 90 on Linux
I didn't do extensive debugging, but I suspect the root cause is somewhere in Wikimedia UI. The button has CSS classes eg-sp-contribute-to-original mw-ui-button mw-ui-progressive mw-ui-quiet. The progressive quiet button in the OOUI demo looks correct, so it doesn't appear to be a case of combining classes that should never be combined.