1. Create 30x30 icon in skectch.
2. Export to SVG
3. Optomize the icon using https://jakearchibald.github.io/svgomg/
4. Take the optomize file and Compress it using https://websemantics.uk/tools/svg-to-background-image-conversion/
5. Add to css. 
6. Include !important
7. Include background-repeat: no-repeat;

.mapboxgl-ctrl-icon.mapboxgl-ctrl-zoom-in {
  background-image: url("data:image/svg+xml;charset=utf8,%3Csvg width='30' height='30' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cpath d='M14 14V9h2v5h5v2h-5v5h-2v-5H9v-2h5z' fill='%23A0A7B4'/%3E%3Cpath d='M5 5h20v20H5z'/%3E%3C/g%3E%3C/svg%3E") !important;
  background-repeat: no-repeat;
}

.mapboxgl-ctrl-icon.mapboxgl-ctrl-zoom-out {
  background-image: url("data:image/svg+xml;charset=utf8,%3Csvg width='30' height='30' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cpath d='M5 5h20v20H5z'/%3E%3Cpath fill='%23A0A7B4' d='M9 14h12v2H9z'/%3E%3C/g%3E%3C/svg%3E") !important;
  background-repeat: no-repeat;
}

.mapboxgl-ctrl-icon.mapboxgl-ctrl-compass > .mapboxgl-ctrl-compass-arrow {
  width: 30px;
  height: 30px;
  margin: 0px;
  background-image: url("data:image/svg+xml;charset=utf8,%3Csvg width='30' height='30' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cpath d='M5 5h20v20H5z'/%3E%3Cpath d='M15 26.314L19.243 15 15 3.686 10.757 15 15 26.314zm0-2.849L12.2 16h5.6L15 23.465z' fill='%23A0A7B4' fill-rule='nonzero'/%3E%3C/g%3E%3C/svg%3E") !important;
  background-repeat: no-repeat;
}

.mapboxgl-ctrl-icon.mapboxgl-ctrl-fullscreen {
  background-image: url("data:image/svg+xml;charset=utf8,%3Csvg width='30' height='30' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cpath d='M5 5h20v20H5z'/%3E%3Cpath d='M19 7h5v5h-2V9h-3V7zm-8 0v2H8v3H6V7h5zm8 16v-2h3v-3h2v5h-5zm-8 0H6v-5h2v3h3v2z' fill='%23A0A7B4'/%3E%3C/g%3E%3C/svg%3E") !important;
  background-repeat: no-repeat;
}

Position the buttons:

          this.map.addControl(
            new mapboxgl.NavigationControl({
              // Hide rotation control.
              showCompass: true
            }),
            "top-right"
          );
          
          this.map.addControl(
            new mapboxgl.FullscreenControl({
              // Full Screen Control.
            }),
            "bottom-right"
          );