Create a custom control to change the page theme

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Hi.
Anyone knows how can I change the theme from my custom control.
My custom control render a dropdownlist with the availables themes, getting
in the Themes folder.
But when I try to chane the Page.Theme property, I get this error:

The 'Theme' property can only be set in or before the Page_PreInit event

So, how can I set the Theme in or befor the Page_PreInit?

Thanks.