There are 4 different themes you can adjust in Xfce: the window decorations, the GTK+ interfaces, the cursors, and the icons. You can find themes on various website, there are a couple of them:

• Xfce Look: GTK+, icons, cursors and window manager themes
• Xfce Forums: mostly window manager themes
• Gnome Art: icons and GTK+ themes
• Deviant Art: window manager themes
• Deviant Art: GTK+ themes only

• Extract the theme in ~/.themes
  • System wide installation in ${sysprefix}/share/themes
• Make sure the directory layout looks like this: ~/.themes/<theme_name>/xfwm4/
• If the theme contains scalable images (~/.themes/<theme_name>/xfwm4/*.svg) it's better to install the librsvg package
• Select the theme in the Window Manager Settings

You can find more information about how to create a window manager theme here.

A GTK+ theme is the theme of the toolkit (buttons, textfields, etc). Most of the GTK+ themes need their own theme engine, make sure you have that theme engine installed with the same prefix as the GTK+ toolkit. To install and use a theme, follow these steps:

• Extract the theme in ~/.themes
  • System wide installation in ${sysprefix}/share/themes
• Make sure the theme contains the following file: ~/.themes/<theme_name>/gtk-2.0/gtkrc
• Select the theme in the User Interface Settings

• Extract the theme in ~/.icons
  • System wide installation in ${sysprefix}/share/icons
• Make sure the directory layout looks like this: ./icons/<theme_name>/cursors
• Select the theme in the Mouse Settings. If there is no tab with cursor themes, make sure xfce-mcs-plugins is compiled with Xcursor support

• Extract the theme in ~/.icons
  • System wide installation in ${sysprefix}/share/icons
• Optional: run gtk-update-icon-cache -f -t ~/.icons/<theme_name> to create an icon cache ¹⁾
• If the theme contains scalable icons (~/.icons/<theme_name>/scalable) make sure you have librsvg installed
• Select the theme in the User Interface Settings (2nd tab)

When you have icon theme problems, it's also recommended to install the hicolor-icon-theme package.