You need Menu Backgrounds (by Luffy): https://modworkshop.net/mod/17160

Put the folder in mods/Menu Backgrounds/assets
go to payday 2 options, mod options, menu background, choose "Generic name" as background and that's it.

I recommend using:
"Hotline Miami HUD" (by Eigthan & Dom): https://modworkshop.net/mod/27294
or if you want to keep the original font but make it look better and not so blurry
"HQ inventory fonts" (by Majahet): https://modworkshop.net/mod/16571