There already is a modding tool isn't there? https://github.com/Jayveer/Decima-Explorer Seems to allow you to unpack game files, edit them and then repack them again.
A tool is available that allows you to unpack data from the game: https://github.com/Jayveer/Decima-Explorer I'm not a modder myself so I've no idea whether this will be any help to you, but texture modifications would be awesome!