Bug 503276

Some form-containing documents provide visual clues about the way they want data to be provided, e.g. with or without separators.
With the current solid-colored background used for forms, those clues are hidden to the user, making form filling harder than it should be.

Example:
https://www.formulaires.service-public.fr/gf/cerfa_15763.do

On the 4th line there's a date field. Visual separators exist in the document, and the field length is limited to 8 chars, so the author here expects you to input, for example: 10101990 to indicate the date 10/10/1990. But if you don't see the visual clues because they are hidden by okular's form filling UI, you may be tempted to enter 10/10/90 instead (which IMO is a reasonable way of thinking since you're asked to provide a readable date in 8 chars). And you'd be wrong though, but you'd risk to let this error slip through.

Try to input 10/10/90, then disable the Show Forms mode, and you'll see that it looks wrong:

10|/1|0/90

Where | are the visual separators provided by the PDF to separate dd mm and yyyy.

If the background would've been transparent, you'd have seen that the date fields are going to be separated "naturally", and so you can just enter 10101990.

When using Breeze light the background is white, when using Breeze Dark it's black.

Instead, it should be transparent no matter which theme is currently in use.

Ideally, the form editing preview should reflect the final result, in a WYSIWYG fashion, but I dunno if feasible. Making the background transparent is the next best thing.