
With the transition to each more visually animated desktop theme and graphical user interface, it is easy to forget how quickly the themes that we use now have evolved. So here are some retro desktop themes in the event that all modern themes didn't make you forget about BeOS.
Windows 2000
Let's start with this boring gray Windows theme. If you prefer, you can almost completely recreate it in Ubuntu using the Gnome Redmond theme, a similar window manager and icon set.

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Sequential installation instructions - hereNeXTSTEP
NeXTSTEP was a promising UNIX-like operating system, released in 1988 and formally completed its history in 1996. NeXTSTEP is best known as the ancestor of the somewhat famous Mac OS X.
NeXTSTEP had a very distinctive appearance. So characteristic that it served as the basis for a half-port project, called GNUStep, and created in order to bring the look of a long-forgotten operating system to Linux.
Required files and additional information are here.BeOS
Almost
20-year-old BeOS had an uncluttered design and graphical user interface developed on the principles of clarity and purity, and for many BeOS fans (for example, for the Haiku project) the graphical interface remains a distinctive part of the BeOS heritage.
Download - hereAmigaOS
AmigaOS 4, probably, should not be referred to as “retro”, because the last update of this operating system was just 56 days ago. However, AmigaOS is no more than a nostalgic interest of the majority.

This is the GTK version of the "zami" AmigaOS 4 theme.
Download - hereOS / 2
IBM's OS / 2 — initially developed in collaboration with Microsoft — was a very striking phenomenon. Once it was even called the operating system of the future. But she did not become one, losing the competition to Windows. OS / 2 had a good graphical interface for its time and for those who remember OS / 2 - its GTK version is lower.
Download - hereAfterword
The author really wanted to include the old school Acorn in this list, but he did not find the corresponding GTK theme. Similarly for the classic Mac OS theme.