Submit button on home page form stopped showing (Avenue template)

I'm managing a simple site (7.0 with the Avenue template). I've used the option to have a custom form for submissions on the home page. But some weeks ago the Submit button stopped showing up on the pop up form. 

I'm using a form on another page in this site, and there it works fine. In both cases I use the option to send the submissions to an email address. 

I tried to put a form on the homepage of another 7.0 site with the Avenue template and here it works.

Confusing! Does anyone have an idea on what's wrong?

I've experimented somewhat: I changed the font color for buttons to a light grey. And now you can see and click on the text "SKICKA" (Send) but the background color for the button does not show.

Thanks! Worked!