This thread (and a number of others that can be found via search) is worth a read if you want to customize the WebListBox’s appearance:
For the sake of brevity, though, adding this to App.HTMLHeader should make WebListBox backgrounds transparent:
<style>
.XojoListBox .DTFC_ScrollWrapper,
.XojoListBox .table,
.XojoListBox .dataTables_wrapper tr.buffer,
.table-striped tbody tr:nth-of-type(odd) {
background-color: transparent !important;
}
</style>