/* input(1,1): run-time error CSS1019: Unexpected token, found '/'
input(1,8): run-time error CSS1031: Expected selector, found '/'
input(1,8): run-time error CSS1025: Expected comma or open brace, found '/'
input(18,1): run-time error CSS1019: Unexpected token, found '<'
input(19,10): run-time error CSS1030: Expected identifier, found 'addEventListener('
input(19,10): run-time error CSS1031: Expected selector, found 'addEventListener('
input(19,10): run-time error CSS1025: Expected comma or open brace, found 'addEventListener('
input(26,2): run-time error CSS1019: Unexpected token, found ')'
input(27,1): run-time error CSS1019: Unexpected token, found '<'
input(27,2): run-time error CSS1019: Unexpected token, found '/'
input: run-time error CSS1031: Expected selector, found ''
input: run-time error CSS1066: Unexpected end of file encountered */
/Themes/YourTheme/Content/css/styles.css
.nivoSlider {
    border: 2px dashed red;
    position: relative;
}

.nivoSlider::after {
    content: "Slider";
    position: absolute;
    top: 3px;
    left: 3px;
    background: red;
    color: white;
    padding: 3px 6px;
    font-size: 10px;
}

<script>
document.addEventListener("DOMContentLoaded", function() {
    var slider = document.querySelector('.nivoSlider');
    if(slider){
        var w = slider.offsetWidth;
        var h = slider.offsetHeight;
        console.log("Slider grootte: " + w + " x " + h);
    }
});
</script>