I made this thread because I know some people like myself, might not know about other threads in other sections, that still might apply to our device, the S4...
Open up TWRP folder on internal sd card
Create folder named theme (case sensitive)
Download any of these zips, change its name to ui.zip
Move the ui.zip file to the theme folder you just created
Restart into recovery.
I'll just link to the threads I've found with custom twrp themes. Follow my instructions and it'll work fine. Some of these are different sizes, but they all look fine on the s4.
This one is an aroma installer. I'm not sure if anyone wants to try using it with the s4, but you can extract the theme zips from inside, and then follow my steps above.
@The Sickness created this amazing thread
You've Just Been Tapatold ♧♢dbombROMv3.4♤♡
My SGS4 Theme ( Taking Req. )