Modifying particular items can be done just by modifying text files. Go to data/json/items/ and edit the relevant files in any text editor. You want to lower the material thickness values. There’s a ton of them, so you’d better have a good way of doing so (unless you only want to nerf a small number of items).
Or you can recompile the game and modify values globally. I recommend the second option, even though it requires non-zero computer skills.
The learning speed change would require recompiling the game.
First get the source (preferably with git, so that it’s easier to update, but you can just get the zipped source here: https://github.com/CleverRaven/Cataclysm-DDA/archive/master.zip )
Then get the tools - I heard most people use CodeBlocks to compile the game.
Unpack the source and open the CacaclysmWin.cbp in CodeBlocks.
Modify the game. The parts of code you’d want to change are in src/player.cpp (player::adjust_for_focus) and src/item.cpp (item::bash_resist and cut_resist).
Since you’d be introducing very simple numeric changes, that wouldn’t require any real programming skills. Just modify the formula after “return” at the end of the block of code.
Then build the game. It can easily take an hour, but doesn’t require you to look at it constantly.