Icons

The freedesktop project provides the Icon Theme Specification, which applies to most linux desktop environments and tries to unify the look of a whole bunch of icons in an icon-theme. Freedesktop also provides the Icon Naming Specification, which defines a standard naming scheme for icons believed to be installed on any system. The default theme hicolor should include them all.

Installation

Icons and emblems

To append a custom icon to an existing icon theme xdg-icon-resource can be used. This will resize and copy the icon to $HOME/.local/share/icons/. With this method, custom emblems can also be added. Examples:

$ xdg-icon-resource install --size 128 --context emblems archuser-example.png # add as emblem
$ xdg-icon-resource install --size 128 archuser-example.png # add as normal icon

Mime type icons

Today's file managers do not rely on the traditional mime type which file --mime outputs. Instead definitions from /usr/share/mime/ are used. Calling an icon according to the definition found there and copying it to ~/.local/share/icons will cause the file manager to display the custom mime type icon. This command illustrates the method:

Creates a custom icon for keepass database files (*.kdb)

# grep kdb /usr/share/mime/globs | egrep -o '.+\/[^:]+' | tr '/' '-'
application-x-keepass ;# rename your icon according to this output
xdg-icon-resource install --size 128 --context mimetypes application-x-keepass.png

Icon themes

Tip: It is recommended to install the hicolor-icon-theme package as many programs will deposit their icons in /usr/share/icons/hicolor and most other icon themes will inherit icons from the Hicolor icon theme.

From a package

Official repositories — "icon-theme" search.
AUR — "icon-theme" search.

Manually

If you cannot find a package for the icon theme you are looking for, you will need to install it manually.

Firstly, find and download your desired icon pack. Many different icon themes can be downloaded from the following sites: Opendesktop.org and Xfce-look.org.

Then navigate to the directory which contains the icon pack and extract it. Example tar -xzf /home/user/downloads/icon-pack.tar.gz.

Move the extracted folder containing the icons to either ~/.icons or ~/.local/share/icons (user only) or to /usr/share/icons (systemwide).

Optional: run gtk-update-icon-cache -f -t ~/.icons/<theme_name> to update the icon cache.

Select the icon theme using the appropriate configuration tool for your desktop environment or window manager.

fstab / gvfs

According to this document file managers using GVFS (like GNOME Files or Thunar) can display icons for custom locations, like NFS shares. All you need are some extended mount options inside /etc/fstab with icon names supported by your selected icon theme:

/etc/fstab

hostname:/ /mnt/ nfs4 defaults,_netdev,user,rw,exec,comment=x-gvfs-show,x-gvfs-name=Network%20Attached%20Storage,x-gvfs-icon=network-server,x-gvfs-symbolic-icon=network-server,timeo=14,noatime 0 0