omf.theme: Validate if theme is installed

pkg/omf/cli/omf.theme.fish

@@ -1,4 +1,10 @@
 function omf.theme -a target_theme
+  if not contains "$target_theme" (omf.packages.list --installed --theme)
+    echo (omf::err)"Theme not installed!"(omf::off)
+    echo Install it using (omf::em)omf install $target_theme(omf::off)
+    return $OMF_INVALID_ARG
+  end
+
   set -l current_theme (cat $OMF_CONFIG/theme)
   test "$target_theme" = "$current_theme"; and return 0
 
@@ -10,7 +16,7 @@ function omf.theme -a target_theme
   if not omf.check.fish_prompt
     echo (omf::err)"Conflicting prompt setting."(omf::off)
     echo "Run "(omf::em)"omf doctor"(omf::off)" and fix issues before continuing."
-    return 1
+    return $OMF_INVALID_ARG
   end
 
   # Replace autoload paths of current theme with the target one