I hacked this together the other day. Replace /usr/local/www/css/login.css contents with this (has to be done after each pfSense update):

body, html {
    height: 100%;
    padding: 0;
    margin: 0;
}

body {
    width: 100%;
    background-color: #333333;
}

header {
}

#headerrow {
    position: fixed;
    height: 90px;
    top: 0;
    width: 100%;
    background-color: #333333;
}

.pagebody {
    position: absolute;
    top:0px;
    bottom:25px;
    width: 100%;
    color: white;
}

.pagebodywarn {
    position: absolute;
    top:140px;
    bottom:25px;
    width: 100%;
    color: white;
}

.nowarning {
        height: 80px;
        padding-top: 10px;
}

#hostspan {
        text-align: right;
}

.msgbox {
        padding-right: 60px;
        padding-top: 25px;
}

@media only screen and (max-width : 768px) {
    /* only size 'xs' and below */
    #headerrow {
        height: 100px;
    }

    .pagebody {
        top: 0px;
    }

    .pagebody2 {
                top: 250px;
    }

    .nowarning {
                height: 60px;
        }

        .msgbox {
                padding-right: 0px;
                padding-top: 0px;
        }

        #hostspan {
                text-align: center;
        }
}

#footertext {
    position: fixed;
    height: 0px;
    bottom: 0;
    width: 100%;
    background-color: #343434;
    color: #333333;
    text-align: center;
}

.loginCont {
    position: absolute;
    top: 50%; left: 50%;
    transform: translate(-50%,-50%);
    height: 55%;
    width: 80%;
}

.error-panel a {
        color: white;
}
p.form-title
{
    font-family: 'Open Sans' , sans-serif;
    font-size: 20px;
    font-weight: 500;
    text-align: center;
    color: #FFFFFF;
    margin-top: 5%;
    text-transform: uppercase;
    letter-spacing: 4px;
}

form.login
{
    width: 270px;
    margin: 0 auto;
}

form.login input[type="text"], form.login input[type="password"]
{
    width: 100%;
    margin: 0;
    padding: 10px 10px;
    background: 0;
    border: 0;
    border-bottom: 1px solid #FFFFFF;
    outline: 0;
    font-size: 18px;
    font-weight: 600;
    letter-spacing: 1px;
    margin-bottom: 5px;
    color: #FFFFFF;
    outline: 0;
}

form.login input[type="submit"]
{
    width: 60%;
    font-size: 14px;
    text-transform: uppercase;
    font-weight: 500;
    margin-top: 36px;
    outline: 0;
    cursor: pointer;
    letter-spacing: 1px;
    margin : 0 auto;
    margin-top: 36px;
    display: block;
    background-color: #333333;
}

form.login input[type="submit"]:hover
{
    transition: background-color 0.5s ease;
}

form.login label, form.login a
{
    font-size: 12px;
    font-weight: 400;
    color: #FFFFFF;
}

form.login a
{
    transition: color 0.5s ease;
}

form.login a:hover
{
    color: #2ecc71;
}

.logoCol {
    height: 100%;
}

/** Re-style web-kit broswer autocomplete boxes (Fixes Chrome's ugly yellow background) **/
@-webkit-keyframes autofill {
    to {
        color: white;
        background: transparent;
    }
}

input:-webkit-autofill {
    -webkit-animation-name: autofill;
    -webkit-animation-fill-mode: both;
}