.custom-reviews-section .review-box {
  background-color: #f5f5f5 !important;
}
.custom-reviews-section * {
  background-color: #f5f5f5 !important;
  color: #333 !important;
}
<style>
  .custom-reviews-section {
    background-color: #f5f5f5 !important;
  }
</style>