I’m not sure that is possible with the Gravity Forms dropdown field. However, if you will enable the enhanced user interface for your dropdown and use the CSS below, I believe you will have what you are wanting. Here is the screenshot of the expected output:
And here is the CSS:
.gform_wrapper.gravity-theme .chosen-container-single .chosen-search {
display: none;
}
.gform_wrapper.gravity-theme .chosen-container .chosen-results li.highlighted {
background: #1b1a1a;
color: #357b1d;
}
.gform_wrapper.gravity-theme .chosen-container-single .chosen-single {
background: none;
background-clip: none;
box-shadow: none;
border-color: black;
}
.gform_wrapper.gravity-theme .chosen-container-active.chosen-with-drop .chosen-single {
background: none;
background-clip: none;
box-shadow: none;
}
.gform_wrapper.gravity-theme .chosen-container-active.chosen-with-drop .chosen-single {
background: none;
background-clip: none;
box-shadow: none;
border-style: none;
border-bottom-style: solid;
border-bottom-width: 3px;
border-bottom-color: #111111 !important;
margin-bottom: 10px
}
.gform_wrapper.gravity-theme .chosen-container .chosen-results li.active-result {
font-size: 20px;
padding: 10px;
}