plugins/themes:

Rewrite theme() and lstheme() to remove pwd and variable side effects and make behavior consistent.

Include custom themes in lstheme() output.
Add comments that lay down the function interface.
Look for custom themes in $ZSH_CUSTOM/themes as well as base $ZSH_CUSTOM.
Rewrite theme's random theme selection in terms of lstheme() so random themes are included, and to streamline code.
Add a failure indication to theme() with nonzero return status. When a bad theme name is requested, indicate so by issuing a warning and returning nonzero.

plugins/themes/themes.plugin.zsh

@@ -1,24 +1,55 @@
-function theme
-{
-  if [ -z "$1" ] || [ "$1" = "random" ]; then
-    themes=($ZSH/themes/*zsh-theme)
-    N=${#themes[@]}
-    ((N=(RANDOM%N)+1))
-    RANDOM_THEME=${themes[$N]}
-    source "$RANDOM_THEME"
-    echo "[oh-my-zsh] Random theme '$RANDOM_THEME' loaded..."
+# Load a theme
+# 
+# Usage:
+#   "theme <name>" loads the named theme
+#   "theme random" or "theme" with no argument loads a random theme
+# Return value:
+#   0 on success
+#   Nonzero on failure, such as requested theme not being found
+function theme() {
+  if [[ -z "$1" ]] || [[ "$1" = "random" ]]; then
+    # Select a random theme
+    local themes n random_theme
+    themes=($(lstheme))
+    n=${#themes[@]}
+    ((n=(RANDOM%n)+1))
+    random_theme=${themes[$n]}
+    echo "[oh-my-zsh] Loading random theme '$random_theme'..."
+    theme $random_theme
   else
-    if [ -f "$ZSH_CUSTOM/$1.zsh-theme" ]
-    then
-      source "$ZSH_CUSTOM/$1.zsh-theme"
-    else
-      source "$ZSH/themes/$1.zsh-theme"
+    # Main case: load named theme
+    local name theme_dir found
+    name=$1
+    found=false
+    for theme_dir ($ZSH_CUSTOM $ZSH_CUSTOM/themes $ZSH/themes); do
+      if [[ -f "$theme_dir/$name.zsh-theme" ]]; then
+        source "$theme_dir/$name.zsh-theme"
+        found=true
+        break
+      fi
+    done
+    if [[ $found == false ]]; then
+      echo "[oh-my-zsh] Theme not found: $name"
+      return 1
     fi
   fi
 }
 
-function lstheme
-{
-  cd $ZSH/themes
-  ls *zsh-theme | sed 's,\.zsh-theme$,,'
+# List available themes by name
+#
+# The list will always be in sorted order, so you can index in to it.
+function lstheme(){
+  local themes x theme_dir
+  themes=()
+  for theme_dir ($ZSH_CUSTOM $ZSH_CUSTOM/themes $ZSH/themes); do
+    if [[ -d $theme_dir ]]; then
+      themes+=($(_omz_lstheme_dir $theme_dir))
+    fi
+  done
+  echo ${(F)themes} | sort | uniq
 }
+
+# List themes defined in a given dir
+function _omz_lstheme_dir() {
+  ls $1 | grep '.zsh-theme$' | sed 's,\.zsh-theme$,,'
+}