Silence output from update-alternatives and fix syntax error

scripts/paint-it-black.sh

@@ -41,6 +41,7 @@ sedfilter \
 
 notice "You may want to run 'update-initramfs -u -k all && update-grub' now :)"
 
-if update-alternatives --query gdm3.css | grep '^Link: ' | grep -q '/ubuntu.css$'
+if update-alternatives --query gdm3.css 2>/dev/null | grep '^Link: ' | grep -q '/ubuntu.css$'
+then
 	notice "To modify the background of gdm, check 'update-alternatives --config gdm3.css'"
 fi