Theseus Services is a content mod providing a number of assets for Arma 3, aimed at Private Military Company game style, developed around the needs of Theseus Incorporated community. Starting out as a mod by Pomigit, then maintained by BadHabitz with a number of other community developers contributing, it is a re-brand of a highly successful PG Services mod.
The project is entirely open-source and all contributions are welcome. Feel free to maintain your own custom version, so long as the changes you make are open to the public in accordance with the GNU General Public License (GPLv2).
Reference the wiki to learn about ways of contributing, getting sources and general information about the mod.