How to change a DIV padding without affecting the width/height ?

Declare this in your CSS and you should be good:

* { 
    -moz-box-sizing: border-box; 
    -webkit-box-sizing: border-box; 
     box-sizing: border-box; 
}

This solution can be implemented without using additional wrappers.

This will force the browser to calculate the width according to the “outer”-width of the div, it means the padding will be subtracted from the width.