The only major issue I see is that the keyboard focus indicator is not adequate. You are relying on the browser default for this and on my Firefox that default is a 1px dotted blue outline that I can’t see at all on the background color you are using. I always customize the focus indicator because the browser defaults are almost never enough.
One minor suggestion I would make is to get rid of the CSS width on div#form-background and use max-width instead (in
em units). Doing this you don’t even need a break point for responsiveness. And using
ems will allow the width to grow as the font size increases.
But yes, I agree with @Roma, this is very good. Nice job!