add_rgb.js

// Usage: 
// var bgcolor = $("#element-id").css("background-color");
// var darker_bgcolor = addRGB(bgcolor, -50);
function addRGB(rgb, n) {
    var spl = rgb.split(",");
    var r = spl[0].replace("rgb(", "");
    var g = spl[1].replace(" ", "");
    var b = spl[2].replace(" ", "").replace(")", "");

    r = r * 1 + n;
    g = g * 1 + n;
    b = b * 1 + n;

    if (r > 255) r = 255;
    if (g > 255) g = 255;
    if (b > 255) b = 255;

    return "rgb("+r+","+g+","+b+")";
}