index.css

#canvasPreview {
  margin: 0px !important;
  background: black;

  image-rendering: optimizeSpeed; /* Older versions of FF */
  image-rendering: -moz-crisp-edges; /* FF 6.0+ */
  image-rendering: -webkit-optimize-contrast; /* Safari */
  image-rendering: -o-crisp-edges; /* OS X & Windows Opera (12.02+) */
  image-rendering: pixelated; /* Awesome future-browsers */
  -ms-interpolation-mode: nearest-neighbor; /* IE */
}

#panzoom-parent {
  background: black;
}

pre {
  margin-bottom: 0;
}

.accordion-button:not(.collapsed) {
  background-color: #eeeeee;
}

.accordion-button:focus {
  border-color: #eeeeee;
  box-shadow: 0 0 0 0.25rem hsla(0, 0%, 75%, 0.25);
}

.accordion-button:not(.collapsed)::after {
  background-image: url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' fill='%23212529'%3e%3cpath fill-rule='evenodd' d='M1.646 4.646a.5.5 0 0 1 .708 0L8 10.293l5.646-5.647a.5.5 0 0 1 .708.708l-6 6a.5.5 0 0 1-.708 0l-6-6a.5.5 0 0 1 0-.708z'/%3e%3c/svg%3e");
  transform: rotate(-180deg);
}