where is the default folder for theme, icons, and wallpaper?

Solution 1:

Wallpaper

The default wallpaper directory is found at /usr/share/backgrounds/ and the default wallpaper is called warty-final-ubuntu.png so overwrite it to change the default wallpaper easily.

Icons

For Icons globcally it's the directory /usr/share/icons/ which typically contains pre-installed themes (shared by all users)

Or locally in your home folder : ~/.icons which typically contains folders with the themes installed by the user.

Also, many applications have their icons in /usr/share/pixmaps/

Themes

For Themes globally it's the directory /usr/share/themes/ which typically contains default themes (shared by all users)

Or locally in your home folder : ~/.themes which typically contains themes installed by the user.