Upgrades Plugin - Upgrade packages for themes

I am trying to use the "Upgrades Plugin" to allow customers to purchase "Premium Themes/Templates". I am having problems with activating/deactivating themes.

Does anyone know how this can be done?

I created my own plugin called premium-themes.php
And the code looks a little like this:

$upgrade_premium_themes = md5("premium-themes":wink:;
upgrades_register_feature("$upgrade_premium_themes", "Premium Themes", "An auto detect plugin that will allow for easy integration with Premium Themes.":wink:;

if(upgrades_active_feature("$upgrade_premium_themes":wink: == "active":wink:{
// This is where I am having problems....
// I cant figure out how to activate/deactivate a theme.
}

I have tried using functions like:
update_site_option(); but everything I do doesn't seem to work.