You research techs to increase ship customisaiton. You have the jewellery and everything right off the bat, but you research better weapons, better defences, better (bigger) hulls, better life support, better engines, and certain special modules (namely invasion, construction, colonisation, and trade. In the expansion there will also be mining modules).
You can make your own hulls as .x files, and upload them. The game will recognise and import them. https://forums.galciv2.com/?ForumID=162&AID=117337 is a link to a star-trek mod, where ships have been made and then imported into the game.
You can customise your races dialogue, abilities, history, pictures, anything you want. The data used in modding is all done in the .xml format, which is the basic format of wordpad, so instantly moddable on any windows computer. You can change any of 26 statistics (a few more are currently unused, may be unlocked in the expansion). You can mod the races you play against. You can also mod the United Planets, and make your own issues in the .xml files, which get used in-game.
All technology in game is moddable, as are political parties and weapons. As well as being moddable, you can make your own for all these things.
Basically, as far as moddability goes, this is probably the best game out there.