1) Install Luffy's Menu Backgrounds Mod (Needs BLT)

2) Go to "mods\Menu Backgrounds\Assets" and place the themes you want in that folder

3) Then open the "[Overrides]" folder and choose one of the three folders and place said folder into "assets\mod_overrides"

4) Open the game and go into Options -> Mod Options -> Menu Backgrounds and choose your set of backgrounds
 - If you don't like the images used for the lobby and inventory then make sure the "Use standard texture for all menus" box is ticked on, this will only load the standard.targa 
   file inside the folder, you may delete the rest if you only plan on using the standard texture.

5) That's about it