mana/sass/_duo.scss

.#{$ms-prefix}-duo {

    // base border styles
    &::after {
        color: #000;
        content: '';
        display: block;
        font-size: 1em;
        height: 100%;
        left: 0;
        position: absolute;
        top: 0;
        width: 100%;
    }

    // set default inner to black unless we're adding color to the background
    color: #fff;

    // multicolor symbol
    &.#{$ms-prefix}-multicolor::after {
        content: "\e986";
    }

    // colorized versions
    &-color {
        &::before {
            color: $ms-border-black;
        }
        &.#{$ms-prefix}-multicolor {
            &::after {
                color: $ms-multicolor-flat;
            }
            &.#{$ms-prefix}-grad::after {
                background: $ms-multicolor-grad;
                -webkit-background-clip: text;
                -webkit-text-fill-color: transparent;
            }
        }
    }

}