Switching CSS classes based on screen size

CSS Media Queries are definetly the way to go.

You can easily separate your CSS based upon the browser size, pixel density, etc.

Here's a list of examples from CSS-Tricks.

/* Smartphones (portrait and landscape) ----------- */
@media only screen 
and (min-device-width : 320px) 
and (max-device-width : 480px) {
    /* Styles */
}

/* Smartphones (landscape) ----------- */
@media only screen 
and (min-width : 321px) {
    /* Styles */
}

/* Smartphones (portrait) ----------- */
@media only screen 
and (max-width : 320px) {
    /* Styles */
}

/* iPads (portrait and landscape) ----------- */
@media only screen 
and (min-device-width : 768px) 
and (max-device-width : 1024px) {
    /* Styles */
}

/* iPads (landscape) ----------- */
@media only screen 
and (min-device-width : 768px) 
and (max-device-width : 1024px) 
and (orientation : landscape) {
    /* Styles */
}

/* iPads (portrait) ----------- */
@media only screen 
and (min-device-width : 768px) 
and (max-device-width : 1024px) 
and (orientation : portrait) {
    /* Styles */
}

/* Desktops and laptops ----------- */
@media only screen 
and (min-width : 1224px) {
    /* Styles */
}

/* Large screens ----------- */
@media only screen 
and (min-width : 1824px) {
    /* Styles */
}

/* iPhone 4 ----------- */
@media
only screen and (-webkit-min-device-pixel-ratio : 1.5),
only screen and (min-device-pixel-ratio : 1.5) {
    /* Styles */
}

Take a look at this https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries.

Another way is to attach the resize event some piece of "switch code".

Something like this: http://jsfiddle.net/s5dvb/

HTML

<div id="body" class="limit400">
    <h1>Hey :D</h1>
</div>

CSS

.limit400 h1 { font-size:10px; }
.limit1200 h1 { font-size:50px; }

JS

$(window).on('resize', function() {
    if($(window).height() > 400) {
        $('#body').addClass('limit1200');
        $('#body').removeClass('limit400');
    }else{
        $('#body').addClass('limit400');
        $('#body').removeClass('limit1200');
    }
})

About the frameworks, try http://purecss.io/ or http://getbootstrap.com/

Hope it helps.