Full Screen Start Menu by Archie and Mudskip
To apply these changes you have two options, either pull the branch from the TeamAqua fork of pokeemerald (instructions below), or copy in the changes by hand Here is the diff
By default the Stat Editor when pulled works from the Party Menu, it should be trivial to gate this behind a flag, or call the StatEditor_Init function from an item callback if you want it to be an item. There are also commented-out changes to the start menu in the branch for opening it from there, you can ignore the changes to this file if you don't want it there.
You use git remote add and git pull commands to pull in a feature branch. That is,
git remote add team_aqua https://github.com/TeamAquasHideout/pokeemerald
git pull team_aqua sandbox
(instructions from Pawkkie)