I also totally relate to familiarity and nostalgia as playing a huge part in how comfortable a forum feels. My clean and modern may look really white and sterile to some of our members.
I came up with a solution, worked on it since I woke up this morning, and here we are. I have added a PSF Classic theme that you can select in your board preferences. Go to your User Control Panel:

From there click your Board Preferences tab and you'll find a new entry titled My Board Style, which you can use to select between the PSF Default theme and the PSF Classic Theme:

Selecting the PSF Classic Theme will transform this forum into something that isn't an exact replica of the old PSF, but will probably seem a little familiar:

This new PSF Classic Theme is a work in progress and it has some issues. I'll be working on it over the next week or so as I see stuff that is off and try to implement fixes for any issues. It isn't as pretty on a phone as the PSF Default Theme, but it is usable. Let me know if you have any gripes with it or if you come across any glitches in the Suggestions thread:
viewtopic.php?f=20&t=51
Enjoy!