wagtail/client/scss/components/_loading-mask.scss

// Loading mask: overlays a certain area with a loading spinner and a faded out cover to prevent interaction
.loading-mask {
    &.loading {
        position: relative;

        &:before,
        &:after {
            position: absolute;
            display: block;
        }

        &:before {
            content: '';
            top: -5px;
            left: -5px;
            bottom: -5px;
            right: -5px;
            z-index: 1;
            background-color: rgba(255, 255, 255, 0.5);
        }

        &:after {
            font-size: 30px;
            width: 30px;
            line-height: 30px;
            left: 50%;
            top: 50%;
            margin: -15px 0 0 -15px;
            font-family: wagtail;
            animation: spin 0.5s infinite linear;
            content: '1';
            z-index: 2;
            color: $color-teal;
        }
    }
}