Reason I was asking because it was possible that it was email-specific, but with the link you provided, I was able to determine it was any valid email.
That said, the field is still fully editable, and you could finish the form, but there are 2 details involved that make it appear otherwise.
First up would be a detail on our end. Once HTML/browsers evaluate the email to be valid, we use the pseudo selectors to set a background image of the green but it looks like we also end up setting the background-color to transparent.
background: url("../images/check_circle.svg") no-repeat;
That’s something that we will need to tweak, so that instead of setting that to transparent, we leave it to inherit what’s in the theme already.
One detail on your theme’s end is that the :focus font color is matching the gray background, at least temporarily, making it look like there’s no text. Click away and then re-click and it seems to turn the text black. That said, black on gray is still darn hard to read.
I want to get the transparent override removed from our plugin, so I will be opening an issue for that. In the meantime, with some css specificity and touchups you should be able to get a white background back on the input.
Let me know if you have any other questions or concerns. Hopefully everything above makes sense.
