changing the properties in a external CSS thorugh Javascript -

there's external css file , want change 1 of property of class using js. property "content" in class should dynamically change local directory of images.

    .carousel-cell {      width: 75%;      height: 580px;      margin-right: 10px;      background: #8c8;      border-radius: 5px;      counter-increment: gallery-cell;     }    /* cell number */    .carousel-cell:before {     display: block;     text-align: center;     content: counter(gallery-cell);     line-height: 580px;     font-size: 80px;     color: white;    } 

what changes should done in order take in list of images in local folder using js. 'images' folder contains 8-10 images. in advance help.