wagtail/client/scss/components/_progressbar.scss

.progress {
    border-radius: 1.2em;
    background-color: $color-teal-dark;
    border: 1px solid $color-teal;
    opacity: 0;

    &.active {
        @include transition(opacity 0.3s ease);
        opacity: 1;
    }

    .bar {
        @include transition(width 0.3s ease);
        border-radius: 1.5em;
        overflow: hidden;
        box-sizing: border-box;
        text-align: right;
        line-height: 1.2em;
        color: $color-white;
        font-size: 0.85em;
        background-color: $color-teal;
        height: 1.2em;
        padding-right: 1em;
    }
}