dynamic themes on the fly

[xkeops]

I have a C# web app ... just playing around with master pages, themes
and skins.
I have read Scott's article:
http://odetocode.com/Blogs/scott/archive/2005/12/09/2604.aspx

Still not really clear for me.

How can I apply a theme for a master page design app other than static
in web config:

<system.web>
<pages theme="trucknews" />
</system.web>

I need to make sure all the website pages will get the theme I specify.
So it's about using PreInit event which a master page doesn't have it
and which is the good way of setting a theme dynamically.

Am I right saying that one cannot actually apply a theme to a master
page but to every page using that master page one by one ???? If so, we
should have a base class which will implement the PreInit event and all
the pages will inherit that page so we dont need to write the PreInit
event on each one of these pages. If I am right so far ... who knows
.... back to my initial question how could I set the theme dynamic ?

I mean ok, I have a base class B and all my pages inherit B. The B
class is implementing PreInit event where I specify the theme but isn't
this static also?

Forgive my confusion. Thanks.

XK