How stretch full browser width and height of a theme?

Hi I have this big problem I have bought a slider template that can be add to any wp page it has the feature to be full horizontal, so if I aneable it the page conteiner of my template keep it to 960 the max allowed for page.

I know that the trick of all this is to find on the css style of the theme where to add modification like this:

#your-slider-selector {
position: absolute;
left: 0;
top: 0;
right: 0;
bottom: 50px; /* height of thumbs (if not required set to 0) */
height: auto;
width: auto;
}

my question is where I should add those modification there are many css style file ….