I'm not quiet sure, what you mean by that. Let me try.
- You want to set a background image
- The login form should be invisible, in order to see the background image and nothing else
- The fields and labels in the form should always be at 100% that they would stay visible, no matter was transparecy the form container has
Right? Dunno whether I can do it. And especially not, how fast. I have a lot of work with a new plugin, I'm writing at the moment. And I don't change anything on the login screen itself actually. It's just a bit of CSS that I inject in the site. I will have to figure out, how everything really works.