The game is indeed written in C# and there is no code obfuscation or anything like that applied, so it should be pretty easy to just decompile it.
If you do modding, I would just ask you to not release any decompiled source code since I have no idea about what kind of licensing implications that has, and also if your mod makes the game multiplayer incompatible with the non-modded game, please change the ResourceContext.NetworkVersion and ResourceContext.NetVersion variables (just add some high number to them), that way there shouldn't be any multiplayer issues with people joining modded games and stuff.