On Debian XFCE the most common locations for storing .desktop entries are:

• /usr/share/applications/
• ~/.local/share/applications
• /usr/local/share/applications

In Debian with XFCE, theme files for the graphical interface are stored in system-wide locations.

  GTK Themes: /usr/share/themes
  Icon Themes: /usr/share/icons
  Window Manager (xfwm4) Themes: /usr/share/themes 

  GTK Themes [modern]: ~/.local/share/themes
  GTK Themes [legacy]: ~/.themes 
  Icon Themes: ~/.local/share/icons or ~/.icons
  Window Manager (xfwm4) Themes: ~/.local/share/themes/ 
  

Download: the theme file from a source like xfce-look.org. Extract: the theme's downloaded archive. Copy: the extracted theme folder (which contains folders like gtk-3.0 for GTK themes or xfwm4 for window manager themes) into one of the user-specific directories mentioned above, such as ~/.local/share/themes/ for GTK/window themes or ~/.local/share/icons/ for icon themes. Select: the new theme by opening Menu > Settings > Appearance and choosing your theme from the Style or Icons tabs.

Select an explicit theme for an application:

env GTK_THEME=Adwaita:dark simple-scan
env GTK_THEME=Nordic-darker simple-scan
env GTK_THEME=Shades-of-gray-Orange simple-scan

env GTK2_RC_FILES="/usr/share/themes/Ambiance/gtk-2.0/gtkrc" firefox

Newer versions of firefox use GTK3. Another way to overrule default thema from within Firefox:

In Firefox URL bar enter about:config. 
   1. Create new String value with name widget.content.gtk-theme-override (Right click > new > String) 
   2. Set value to Adwaita:light (if your distribution does not use GNOME desktop, you should use a name of light GTK theme which is installed on your PC) 
   3. Restart browser 

Login page is created by lightdm. Modify styling by doing:

# nano /etc/lightdm/lightdm-gtk-greeter.conf
--------------------------------------------
Modify the following lines:

theme-name=Adwaita-dark
font-name=Sans Regular 11
xft-antialias=true

To change the icon appearance on the desktop, to: - Show multi line icon text labels - Reduce the space between icon image and text - Change the distance between icons Edit the gtk-3.0/gtk.css file of the theme and add the following lines:

$ nano /.local/share/themes/Shades-of-gray-Orange/gtk-3.0/gtk.css
-----------------------------------------------------------------
XfdesktopIconView.view {
    -XfdesktopIconView-ellipsize-icon-labels: 0;    	/* Allow multi-line text wrapping without truncation */
    -XfdesktopIconView-cell-padding: -4;    			/* Keep cell padding non-negative so bounding boxes do not bleed */
    -XfdesktopIconView-cell-text-width-proportion: 2.5; /* Tighten vertical space specifically between the icon image and the text label */
}
XfdesktopIconView.view .label {
    background: transparent;
}

To restart the XFCE, execute:

$ pkill xfdesktop && xfdesktop &

To start inspector window:

$ pkill xfdesktop && GTK_DEBUG=interactive xfdesktop

• http://wiki.xfce.org/howto/customize-menu
• https://wiki.debian.org/Xfce