[SW] Invert the documents colors in Okular

[NOTE] Here is a newer version of this post:
Invert the document colors in Okular with toggle shortcut

I prefer viewing documents in black background with white text, just as what is usual in the command line environment. It seems no color inverting function in Evince, so I have to use Okular to satisfied my own requirement.

Open your PDF file with Okular, and follow Settings --> Configure okular to open the following dialog.
Check the ``Change Colors'' box and choose ``Invert colors'' of the color mode. Looks great!