[Button with transitions] #CSS_TRICKS

button.scss

.button {
    display: inline-block;
    border: none;
    color: #fff;
    text-decoration: none;
    background-color: $accent;
    padding: 15px 45px;
    font-size: 13px;
    text-transform: uppercase;
    font-weight: 600;
    letter-spacing: 3px;
    border-radius: 2px;
    text-align: center;
    position: relative;
    outline: none;
    transition: background-color .1s ease;
    &::after{
        transition: background-color .2s ease;
        position: absolute;
        content: '';
        height: 4px;
        bottom: 0;
        width: 100%;
        background-color: darken($accent, 50%);
        opacity: .18;
        border-bottom-left-radius: 2px;
        border-bottom-right-radius: 2px;
        left: 0;
    }
    &:focus, &:hover{
        text-decoration: none;
        color: #fff;
    }
    &:hover{
        background-color: lighten($accent, 5%);
        &::after{
            opacity: .22;
        }
    }
    &:active{
        background-color: darken($accent, 5%);
        &::after{
            opacity: .32;
        }
    }
}