Shortcut to negative colors


For those who don't know, there's a simple way to switch to negative colors, that I think it's useful because I use my tablet to read and sometimes it's nicer to read without much brightness, with a dark background. If we go to applications, on widgets, we can add a button on the main screen that allows us to choose negative colors.
But I still want a fastest way to do this, without have to exit the reader Window. I'm looking to some way to add this option to the main buttons on task bar. On the settings we have the option to choose between "print screen","search", and others, but there's no option for "negative colors" or others. I think this is not possible, but I'd like to know if someone know some way to do this.

Thank you