1. What does it mean?
폰꾸 shortens 폰 꾸미기. It can mean arranging wallpapers, icons, and widgets, or decorating a phone case and accessories. Context tells you whether the decoration is on-screen or physical.
Also written or said as ponkku
2. The nuance
It is aesthetic customization, not necessarily changing the phone's operating system. A screenshot of 폰꾸 may simply show a new wallpaper or home-screen layout.
3. Examples
“사진 색에 맞춰 배경화면 폰꾸를 했어.”
I personalized my phone wallpaper to match the photo's colors.
“투명 케이스 안에 그림을 넣어서 폰꾸했어.”
I decorated my phone by putting a drawing inside the clear case.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.