Correctly include the onioncolors package in onion color theme
The onion color theme requires the onion colors to be defined. Without this include, this command:
pandoc -t beamer -V colortheme:onion [...]
.. crashes with:
! Package xcolor Error: Undefined color `OnionBlack'.
By specifying the dependency, we can use only the color theme without importing the rest of the more... elaborate theme which has many more side effects than just changing colors.