If you want 'Add-ons', you should probaly check on the 'Add-Ons and Mods' board.
When it comes to markings, download whatever pack you like, and stuff all the '.png', 'jpg.' etc.. files into your 'FeralHeart>Media>Textures' folder. Then, there should be a '.cfg' file, put that into your main FeralHeart folder, and merge it with the other one :3.
For items, similiar to the one above- place everything but the '.cfg' into 'FeralHeart>Media>Items'. 'items.cfg' goes into the main 'FeralHeart' folder :3,
Edit: Herp-Derp xDD. I Think Lord Suragaha answered your question abit before meh :3.