placeholder-renderer-default.css

.rst__placeholder {
  position: relative;
  height: 68px;
  max-width: 300px;
  padding: 10px;
}
.rst__placeholder,
.rst__placeholder > * {
  box-sizing: border-box;
}
.rst__placeholder::before {
  border: 3px dashed #d9d9d9;
  content: '';
  position: absolute;
  top: 5px;
  right: 5px;
  bottom: 5px;
  left: 5px;
  z-index: -1;
}

/**
 * The outline of where the element will go if dropped, displayed while dragging
 */
.rst__placeholderLandingPad,
.rst__placeholderCancelPad {
  border: none !important;
  box-shadow: none !important;
  outline: none !important;
}
.rst__placeholderLandingPad *,
.rst__placeholderCancelPad * {
  opacity: 0 !important;
}
.rst__placeholderLandingPad::before,
.rst__placeholderCancelPad::before {
  background-color: lightblue;
  border-color: white;
}

/**
 * Alternate appearance of the landing pad when the dragged location is invalid
 */
.rst__placeholderCancelPad::before {
  background-color: #e6a8ad;
}