After changing some settings in the Upfront editor for the Fixer starter theme, this code adds an animation. Fixer: https://premium.wpmudev.org/project/fixer/

upfront-button.css

a.upfront_cta {
    position:relative;
}
a.upfront_cta:after {
    content:'';
    width:100%;
    height:0px;
    background-color:#5ebf6b;
    position:absolute;
    display:block;
    z-index:-1;
    left:0px;
    top:0;
    transition:height .1s linear;
}
a.upfront_cta:hover:after {
    height:100%;
}