metamask-extension/ui/components/app/collectible-default-image/index.scss

.collectible-default {
  background-color: var(--color-background-alternative);
  padding-top: 100%; // retains 1:1 aspect ratio
  position: relative;
  width: 100%;

  &__text {
    overflow: hidden;
    text-overflow: ellipsis;
    text-align: center;
    position: absolute;
    white-space: nowrap;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: calc(100% - 32px);
  }

  &--clickable {
    cursor: pointer;
  }
}