This might not be what you want since you've specified System 7, but if you like the Platinum theme used in Mac OS 8 and 9, I recommend the Platinum9 theme for XFCE, which is what I've been recently using whenever I use a Linux desktop:
I find this theme quite pleasing to use whenever I'm using a Linux desktop. However, it's not yet pixel-perfect, and this also doesn't address the fact that GTK and GNOME applications are written under a completely different set of UI guidelines than classic Mac apps were, and so it's still largely a Mac OS 8-like veneer over the Linux desktop.
Even so, this theme looks like an excellent foundation for a suite of GTK applications that employ the mid-1990s Apple Human Interface Guidelines. What would be tantalizing would be forks of AbiWord and GIMP that look and feel like Microsoft Word 5.1 and classic versions of Photoshop, respectively.
"if you like the Platinum theme used in Mac OS 8 and 9"
There were three iterations (that I recall) of the interface on color Macs - the first one had window controls that were black and white, the second had a subtle 3d effect, and the third (which I think you mean by "Platinum") had more depth and was mostly gray.
I personally preferred the second. Attempts at imitating grayish metal on screen don't appeal to me, and I thought OS X got even worse when they switched to the brushed metal look.
But I really liked the design theme for the physical cases that was also called "Platinum" even though I think it's generally considered an uninspired period for Apple. And I have nothing against the aluminium or titanium Mac cases either.
The one change I would make to a "neo-Mac" window manager would be to have handles on all sides of a window for resizing. Although I like the Mac approach of having scrollbars and a lower right resize control that are substantial and obvious, I also like the ability to resize from all sides like most other interfaces have. I know there were Mac programs for the classic OS that did implement something like that, but it was never universal.
https://github.com/timnetworks/Platinum9
I find this theme quite pleasing to use whenever I'm using a Linux desktop. However, it's not yet pixel-perfect, and this also doesn't address the fact that GTK and GNOME applications are written under a completely different set of UI guidelines than classic Mac apps were, and so it's still largely a Mac OS 8-like veneer over the Linux desktop.
Even so, this theme looks like an excellent foundation for a suite of GTK applications that employ the mid-1990s Apple Human Interface Guidelines. What would be tantalizing would be forks of AbiWord and GIMP that look and feel like Microsoft Word 5.1 and classic versions of Photoshop, respectively.