.app-o-epic-bunting {
  width: 100%;
  height: 50px;
  overflow: visible;
  border-top: 1px solid govuk-colour("grey-2");
  position: absolute;
  left: 0;

  @include govuk-media-query($until: tablet) {
    display: none;
  }

  .app-o-epic-bunting__bunt {
    background-image: image-url("bunting.png");
    background-color: transparent;
    background-repeat: repeat-x;
    background-position: center top;
    width: 99.9%;
    height: 70px;
    display: block;
  }

  .app-o-epic-bunting__bunt--tinsel {
    background-image: image-url("tinsel.png");
  }
}

.app-o-epic-bunting {
  width: 100%;
  height: 50px;
  overflow: visible;
  border-top: 1px solid govuk-colour("mid-grey", $legacy: "grey-2");
  position: absolute;
  left: 0;

  @include govuk-media-query($until: tablet) {
    display: none;
  }

  .app-o-epic-bunting__bunt {
    background-image: image-url("bunting.png");
    background-color: transparent;
    background-repeat: repeat-x;
    background-position: center top;
    width: 99.9%;
    height: 70px;
    display: block;
  }

  .app-o-epic-bunting__bunt--tinsel {
    background-image: image-url("tinsel.png");
  }
}

Update mixins to use new colours and fallback to legacy ones in compatiblity mode
Update mixins to use new colours and fallback to legacy ones in compatiblity mode
