Dear Louis,
Thank you for posting, and welcome to the AtoM community!
If you are referring to plugins to apply a custom theme, we do not have any tutorials available per se, but AtoM 2.x comes shipped with two themes- the default, called Dominion, and a second called ArchivesCanada. The ArchivesCanada theme was written as a customization of the Dominion theme, with the intention that users could look at the code for each to get an idea of how to write their own.
This would be a great opportunity for other community members to jump in with ideas of how they approached their own plugins and customizations- I hope there are some ideas out there! Let us know if there is anything else we can do to help.
Cheers,