The Default appearance takes its colours from the rest of the site so that it stays in step with it. Those colours change when a device is set to dark mode — but the form's card had been fixed to white, so the text turned pale while the card behind it stayed light, leaving it very hard to read.
The card now changes with everything else, so the Default appearance is legible whichever mode the person filling in the form is using. Paper, Blue and Dark set their own fixed colours and were never affected.