Some of these old style themes are interesting for nostalgic reasons but then after a while you do realize that there are reasons why the world has moved on.
But I feel we need only point to Microsoft Word as an example of how the statement.
>Everything was so clear, neat, and discoverable back then
just isn't true. The only difference now is that we have dozens of pieces of software in our lives now instead of the half dozen we used in the 90s, and we don't have one company monopolistically decreeing what the status quo of UI design is.
Meaningful competition is also why I think there is no WIMP in mobile. And that's a double-edged sword.
Press some hot key, and then type the name of your command in a pop up window with full-text search, autocomplete and a hint what the hot key and toolbar icon for that particular command is. Probably single most useful thing that became mainstream roughly after the ubiquitous CLI-phobia mostly went away.
This is probably a niche opinion but I think we have been steadily devolving and going backwards in terms of user design. We waste so much space, require so many clicks for worse usability than we started with. With the death of skeumorphism and textual UIs we regularly have to guess or hover buttons to find out what they are. Even scrollbars are starting to be unusable - I can barely see some of them because they are small, low-contrast and auto-hiding.
I can theme some things (unless some modern packaging tech makes the app ignore my theme), I can make the ones in Firefox usable but ugly (with widget.gtk.overlay-scrollbars.enabled and widget.non-native-theme.scrollbar.style), and so on. But I can see where we are heading. Scrollbars are one of many UI features that were too useful, so they must be destroyed.
I did not expect to feel so old at such a young age.
TL;DR millennial shakes fist at cloud
I want the internet back where everything looked like that phpbb stuff. You just got the information then. Now you have to solve the CCS/JS puzzle first.
I think it includes smaller, denser text.
And something about the borders. Chunkier, more ornate, or griadients/light or something.
And then Windows 8 aesthetic came in, suddenly flat and a few colors with very reduced palette of available widgets became en vogue, but at the same time somehow the load and render times of sites bragging to be "simple" and "minimalistic" and "lightweight" soared with megabytes of assets none of your old sites sporting image borders and marble backgrounds ever had.
edit: I've recently been using Chromafiler (https://github.com/vanjac/chromafiler) and I'm realizing we've gone off in completely the wrong direction since the 90s.
On Windows XP, beside the childish theme, the window buttons were very usable.
Ditto with XFCE with some xfwm4 with a round-surface squared buttons on yellow, green and red, you can find it under the last items of the list.
Let's talk about the icons and usability. On overy modern Gnome3/XP icon theme, the icons seem contrastless and very hard to see.
Meanwhile, the Tango icons are still standing a lot on the file manager's white background, with clear outlines.
Altough I must 'konfess' that the Keramik theme with the Noia icons and some handwritting font made reading advanced man pages in Konqueror a lot less "dramatic" and careless, thus, keeping the stress down.