userContent.css

@media (-moz-proton) {
  @-moz-document url("about:home"), url("about:newtab") {
    /** Activity Stream - Search Focus Border: like URL ***********************/
    /* At DarkMode, Color */
    body[style*="--newtab-background-color:rgba(28, 27, 34, 1);"],
    body[style*="--newtab-background-color:rgba(42, 42, 46, 1);"] {
      /* inner */
      --newtab-focus-border: rgba(0, 221, 255, 0.5) !important; /* Original: #B5D3FF, Better color-mix(in srgb, #B5D3FF 70%, transparent) */
      --newtab-focus-border-selected: rgba(0, 221, 255, 0.5) !important; /* Original: #B5D3FF */
    }

    /** Activity Stream - Web Site Icon: full size ****************************/
    .top-site-outer .tile .icon-wrapper {
      width:  100% !important; /* Original: 48px */
      height: 100% !important; /* Original: 48px */
    }
  }
}