When you want to give classes to elements based on the viewport you could write a simple check in JavaScript based on Bootstrap's viewport.
if(screen.width >= bootstrap-viewport) {
$(elements).removeClass('otherClasses');
$(elements).addClass('someExampleClass');
}
You can however use media queries in CSS to give elements specific styling per viewport. This is a cleaner and more maintainable solution. A small example:
.content-area {
line-height: 1.2em;
}
@media(min-width: @screen-desktop) {
.content-area {
line-height: 1.6em;
}
}