When we graduated from beta status the standard blue SE beta theme was replaced with a customized one:
if it is an enlarged version of part of the Pi's PCB
No, it's art.
I think given that we are going to have a green PCB as the site banner it should at least be part of the Pi's PCB.
Oh well -- that discussion took place 6 months ago, phase completed, move on. I honestly wanted the "Raspberry Pi" title in lemon, but I did not get my way with that. I also thought Wilf's suggestion about buttons/menu headers was great but understand the basic structure here is not totally free form. Put another way: If you want your own site, start your own site.
As far as I'm aware, there is no mechanism by way of which to pester the SE folks with occasional cosmetic tweaks for the interface, although if a meta post here dedicated to that attracted enough support and attention, we could discuss it with a Community Manager (who are paid SE staffers; moderators such as myself are just elected volunteers, and we do not have much in the way of any special direct interaction with the SE staff). The first place to start inquiring about that would be Meta SE proper.
However, I'll warn you now that your time is better spent doing other things because the chance of mounting a successful campaign on that front is slim. The design is done. It achieved a relatively high level of support; currently the #1 and #2 ranked questions on Rpi Meta are the graduation announcement and Pawel's site design proposal; notice none of the proposed changes to that design received even 1/6th the number of votes for the proposal itself.