Add this code to the Custom CSS field on the Style tab of your Reviews widget’s settings:
/*Change background of the image container in popup */
[class*="MediaContainer__Container-sc"] {
background-color: red;
}
/*Change background of the review container in popup */
.es-popup-content-container {
background-color: grey;
}
/*Change font size of the review text in popup */
.es-popup-content-container .es-text-shortener {
font-size: 18px;
}
/*Change popup overlay color */
[class*="Popup__PopupInner-sc"] {
background-color:black;
}