Edit CSS of form

In my website:
http://www.forsafe.it/

I want to change the background-color of the button “Submit” (there, it is “Registrati”), but CSS doesn’t seem to work.
I went to style.css of my theme and I added this code:

#form-wysija-2 .wysija-submit-field {
   background-color: red;
}

and I tried also this:

#form-wysija .wysija-submit-field {
   background-color: green;
}

but they don’t work.
Notice that that is a widget, but I have that form also in another page (I used the shortcode).

https://ww.wp.xz.cn/plugins/wysija-newsletters/