This is how I got the theme changer working without installing Launcher Pro.
Get the application "Any Shortcut" from play store
Open the application and enter the following details
Application : com.lge.launcher2
Activity : com.lge.launcher2.theme.LGThemeChooserActivity
Action: [Keep blank]
Intent : [Keep blank]
Then : create, your shortcut is done !
Now open the shortcut and your theme chooser application will fire up.
Download LG themes from LG Smart World application (not installed by default) and start applying themes.
You can look for LG Smartworld apk online ( Can't post links here

)
Cheers!
But frankly, install a third party launcher (Nova Launcher being my favorite), and you'll never want to use the themes on stock launcher. You can have tons of customizations and themes using the third party launcher. Stock launcher is nice and smooth but if you feel adventurous, try custom launchers.