'Adds external CSS to the Pynliner object. Can be "chained".
Returns self.
>>> html = "<h1>Hello World!</h1>"
>>> css = "h1 { color:#ffcc00; }"
>>> p = Pynliner()
>>> p.from_string(html).with_cssString(css)
<pynliner.Pynliner object at 0x2ca810>'

def with_cssString(self, cssString):

    if (not self.style_string):
        self.style_string = (cssString + u'\n')
    else:
        self.style_string += (cssString + u'\n')
    return self
