You can't do it.
If you use the ?styleid=x URL it saves onto a cookie, not into the user table. If you use the modify profile to save it into the user table, and the user uses the same account on another computer, then the style will be permanent - there is no way you can come up with a hack that can distinguish between computers...
One way is to redirect the user according to their browser, or display settings... that is possible, and not hard to do... just use Javascript.
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