If it is not too much of an effort and it means that Stephen001 can show the page off to his colleagues, I'm fully in favour of customisation.
The catch 22 to your argument is. He shows them this website that's all silvery and pretty. Then they go to the URL themselves at home, and the see the site as is pretty much "wtfuzzle is this? Maybe I got the URL wrong."
They go back tomorrow and double check, he shows them that same silvery sleek layout. And they take extra special care and rewrite the domain letter for letter, in big letters, in permanent marker on solid gold paper and return home. Once again to see the site in it's current form.
Unless he buys them memberships and presets their accounts with his sleek layout design, there's no way they'll stick around (if they are how he implies and judgemental of the website itself).
Theme based appearance changes are nothing new to many a professional application out there and even became a topic for hardware ('click and go modules to apply to your hardware to give them a different look and feel).
As for the little I know about the interaction between CSS, JS and HTML, it should be modular, which means that the page would be able to support different customer depended layouts.
I figure that a 'gamer' would have a different preference from a 'developer' and an Animé fan from a RPGer.
If it is not too much of an effort and it means that Stephen001 can show the page off to his colleagues, I'm fully in favour of customisation.