Load themes from `$ZSH_CUSTOM` instead of `$ZSH/custom`

This seems like a simple oversight, everything else uses `$ZSH_CUSTOM`
rather than `$ZSH/custom`.

oh-my-zsh.sh

@@ -65,12 +65,11 @@ then
 else
   if [ ! "$ZSH_THEME" = ""  ]
   then
-    if [ -f "$ZSH/custom/$ZSH_THEME.zsh-theme" ]
+    if [ -f "$ZSH_CUSTOM/$ZSH_THEME.zsh-theme" ]
     then
-      source "$ZSH/custom/$ZSH_THEME.zsh-theme"
+      source "$ZSH_CUSTOM/$ZSH_THEME.zsh-theme"
     else
       source "$ZSH/themes/$ZSH_THEME.zsh-theme"
     fi
   fi
 fi
-