Is there a guide available to modding the game?

It's great that historical rosters are available - but what about team names/logos/etc?

If I wanted to create such a mod, what file format should I use? What file structure? File sizes for graphics? Et cetera...

Right now all I can figure is I make a sandbox league and manually make the changes within, then export said league prior to play for others to share. But it would be good to know how to do it a little more efficiently, and what dimensions to use on the graphics and such.

I second that. I would love to use mods but I am not a computer expert.