Icons, Themes, backgrounds and Fonts Locations

The reason I wrote this is because I see others looking for this locations all the time.
I will add more in time.
Where Are Fonts Stored

Secret Locations:

/etc/fonts/fonts.conf.  

gedit /etc/fonts/fonts.conf

Standard Locations:

/usr/share/fonts  

/usr/local/share/fonts

/home/<username>/.fonts (where <username> is your user name).

Note: that the .font folder is a hidden folder.

Where are Icons and Themes store

/usr/share/themes

/usr/share/icons

/usr/share/pixmaps 

as well as some user-specific directories for the user-created themes:

~/.themes

~/.icons

~/pixmaps are generally used for menu icons.
~/share/icons are generally used for theme icons.\

Where are Login Themes store

/usr/share/gdm/themes

Where are backgrounds store

/usr/share/pixmaps/backgrounds

/usr/share/backgrounds

Where are Grub splash images store

/boot/grub/splashimages

Where do updates go:

/var/cache/apt/archives/

Essential system binaries, e.g., init, route, ifup.

/sbin/

Libraries for the binaries in /usr/bin/ and /usr/sbin/.

/usr/lib/

State information. Persistent data modified by programs as they run, e.g., databases, packaging system metadata, etc.

/var/lib/

Lock files. Files keeping track of resources currently in use

/var/lock/

Log files

/var/log/

Users' mailboxes

/var/mail/

Information about the running system since last boot, e.g., currently logged-in users and running daemons

/var/run/

Temporary files to be preserved between reboots

/var/tmp/

Configuration files for /opt/

/etc/opt/

Configuration files for SGML

/etc/sgml/

Configuration files for XML

/etc/xml/

Virtual filesystem documenting kernel and process status as text files, e.g., uptime, network

/proc/

Source code, e.g., the kernel source code with its header files

/usr/src/

local data, specific to the host

/usr/local/