In POSIX sh, RANDOM is undefined

 $ shellcheck /usr/share/xfce4/settings/appearance-install-theme
 ....
 In /usr/share/xfce4/settings/appearance-install-theme line 109:
    tmpdir="/tmp/tmp.$$.$RANDOM"
                        ^-----^ SC3028 (warning): In POSIX sh, RANDOM is undefined.
 ....

$RANDOM was introduced with ed1083bd, which is 11 years old. So seems to be either not critical, or an unused code-path