Consistently use -r instead of -f for conditional shell sourcing

xdg-config-dir/herbstluftwm/herbstpanel.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 # env
-# Written in 2019-2020 by Lucas
+# Written in 2019-2020,2023 by Lucas
 # CC0 1.0 Universal/Public domain - No rights reserved
 #
 # To the extent possible under law, the author(s) have dedicated all
@@ -10,7 +10,7 @@
 # Dedication along with this software. If not, see
 # <http://creativecommons.org/publicdomain/zero/1.0/>.
 
-if [ -f ~/.config/herbstluftwm/theme ]; then
+if [ -r ~/.config/herbstluftwm/theme ]; then
 	. ~/.config/herbstluftwm/theme
 fi