Currently the forms don’t have settings to customize the dropdown for the background nor radius but using the teaching from this thread, you can customize the dropdown using CSS with combinators.
For the background color of the dropdown you can use this where you name the section with the form form-section
.form-section [data-hook="popover-content"] div{
background-color: khaki !important;
}
The border radius for only the dropdown is a bit trickier and I couldn’t figure out a good way to target just that since it seems to get an custom class automatically assigned by Wix to the form and its dropdown. However, you can target all containers and adjust the borders to your preference. I think calc(6*1px) was what I saw was the default radius for input elements but again, feel free to adjust.
Note, I didn’t like how a dropdown looked with borders at the top so I only used the bottom but I included the code to have it across all four corners commented out:
.form-section div {
border-bottom-left-radius: calc(6*1px) !important;
border-bottom-right-radius: calc(6*1px) !important;
/*border-radius: calc(6px) !important; *//*if you want borders radius across all four corners*/
}
And here’s a link so you can see it in action.