When Netscape 6 was actually released, it came with a new low contrast Modern theme that had depth, and not the flat high contrast one in the betas. It stayed this way as Netscape for some time, and the Mozilla Suite that became SeaMonkey kept this theme as the default for quite some time.
This is as close an approximation as I could make for this theme, with the icons being recreated from scratch as relatively high resolution styles that reflect this theme from the past (with more added later as I can). Images needed to be exaggerated from what the originals were to fit the smaller area of the Vivaldi toolbars.