LESS How to check if max-width is 100% and change background color

Hi I'm just getting started with LESS, CSS, Javascript, and HTML as I a building a website. I was wondering if it was possible to use LESS to check to see if the width of an element is 100% and once it is change the background color. Like a progress bar being completed. If it's not possible would I use Javascript? Where would I define that statement using the example:

.page-level-progress-menu-item-indicator-bar {
        width:0%;
        height:8px;
        background-color:@primary-color;
    }