.screen-center {
    position: fixed;
    left: 49%;
    top: 50%
}

.spinner-border {
    display: inline-block;
    width: 2rem;
    height: 2rem;
    vertical-align: text-bottom;
    border: .32em solid currentColor;
    border-right-color: transparent;
    border-radius: 50%;
    -webkit-animation: spinner-border .75s linear infinite;
    animation: spin .75s linear infinite;
}

.sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0,0,0,0);
    white-space: nowrap;
    border: 0;
}

@keyframes spin {
    0% { transform: rotate(0); }
    100% { transform: rotate(360deg); }
}