The problem is that it's a very dated and awkward layout, and web design hasn't stood still for the last 10 years. It's unclear and it can be hard to find functions. This is bad.
This is why the Raspberry Pi Foundation is working to improve computing education, and make it easier for kids (and adults) to get into programming and become familiar and comfortable with computers.
Doing something different to make the forums simpler, clearer and less noisy is part of this. While it won't be everyone's cup of tea, it will be better for a large number of visitors. It's meant to be encouraging and inclusive. Part of that is simplifying, clarifying and making common tasks easier.
I don't think people object to the default theme due to the layout, it's mainly about the bright and garish colours which make it harder to use. Toning down the stark colours and improving contrast would be a wise move. Until then, I'll stick with prosilver - the layout may not be great but the colour scheme doesn't make my head hurt.
Hopefully you can find some way to compromise on the colours and tone them down, as clearly they polarise opinion which really isn't something the Foundation forum needs. Again, it's not the layout that is the problem - it's the colour scheme which needs to be made easier on the eye.