Site Template question

[TimberFloorAu]

Say you have 100 members ( example )

4 styles.

default
a
b
and c

And you have every forum set to default.

Then some months later, you know...

all use custom styles a b and c but ...

Now you want NEW MEMBERS to be shown custom style C by default...

How do you do this... without... buggering up all other existing members preferred choices..

Now I have done this in the past where in each forum, I have set :

Custom Style for this Forum STYLE C

and it caused no end of bloody problems, as members naviagated from one forum to another and styles changing..

So what is the best way...?

Is there a way of declaring style C as the default style. Or am i going bloody mad lol ?