How to modify CSS in Opera to get inverted browsing

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I'm fairly sure it's possible but i have no idea how to do it.
Seeing as i have a galaxy s, an inverted browser would be really useful for me to save battery on my amoled screen, and i use opera mini all the time.

But i dont know how to modify the CSS (which is what i think you're meant to do) to get inverted colours (so the background is black and the text is white), but the images stay the same.

A few google searches and xda searches have got me no closer in figuring out how, so please help