I have an old post about how to change the document colors in Okular, and it gave no toggle options. Here I post an updated version for you who might be want to invert the colors of your PDF files in Okular (I myself always need this kind of toggling functions).
By the way, my OS is Ubuntu 16.04 and the Okular's version is 0.24.2. Just for your reference.
First, go to ``Settings --> Configure Shortcuts...''. In the setting window, you could search for the wanted options. I used ``color'' to find the ``Toggle Change Colors'' option as follows:
Choose ``Custom''. When you move your mouse to the ``None'' icon and wait for seconds, it popped out the instruction to add your own shortcut keybindings:
I used ``Ctrl+I'' as the shortcut:
After the setting, you can press ``Ctrl+I'' to toggle between the normal and the inverted color scheme: