.select2-container--admin-autocomplete.select2-container {
    border: 1px solid #aaa;
    border-radius: 4px;
}

.select2-container--admin-autocomplete .select2-search--dropdown .select2-search__field {
    background: var(--body-bg);
    color: var(--body-fg);
    border: 1px solid #aaa !important; # change color according to the border color of your select field
    border-radius: 4px;
}

.select2-container--admin-autocomplete .select2-selection--multiple .select2-selection__choice {
	color: white !important;
	background-color: #007bff !important;
}