Many thanks. I have now created a new custom.css  - file with your code -  in the css folder of shaper_helixultimate, but there is no changement:  no Sign Up in Tablet/Smartfone view.
What is the difference between putting this code  in a  (new) custom.css - file  or directly in the Custom CSS - window of Custon Code of Helix Ultimate?
In in the Custom CSS - window of Custon Code of Helix Ultimate  I have:
sp-main-body {
padding: 6px 0;
}
.sp-module.boxed-module {
border: 2px solid lightgrey !important;
box-shadow: 1px 1px 1px 1px lightgrey !important;
padding: 20px !important;
background-color: #fff !important;
border-radius: 12px !important;
}