gtk-icon-theme-set-custom-theme (theme name)

theme -- a gtk-icon-theme object
name -- a string with the icon theme name to use instead of the configured icon theme, or nil to unset a previously set custom icon theme

Sets the name of the icon theme that the icon theme uses overriding system configuration. This function cannot be called on the icon themes returned from the gtk-icon-theme-default and gtk-icon-theme-for-screen functions.