I try to customize KOrganizer.
I would really like a dark theme, but I haven't been able to find one.

I've read in a thread how to change background color and work hour color in the file korganizerrc.

Is there any documentation to the commands that can be used in this file?

You should be able to change the color scheme used by Korganizer through its configuration dialog: Settings -> Configure KOrganizer -> Colors and Fonts.

Insofar as you need to configure the colors outside the use of the configuration dialog (such as scripting, perhaps), the korganizerrc file follows a standard structure and format for config files: sections are denoted by titles in brackets, settings are key=value pairs. Specific sections and entries are application-specific and unlikely to be documented.
I'd suggest starting by changing each of the colors through the configuration dialog so that it will update the file to include all the different colors that can be configured in the [Colors] section of the file. Each line for a given color setting ends with 3 numbers which represent the Red, Green and Blue components of the color, respectively; using a scale of 0-255.
For scripted changes, you might use kwriteconfig to apply the changes.