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)

What happens?:

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.