The very first UNIX GUI programs didn't have any widget kits to play with. X had no concept of a button, checkbox, dial, or even a titlebar. Those were up to the programmers to create. If you didn't implement your own button, you didn't have any buttons. This led to a tremendous amount of inconsistency from program to program, making everything look weird and out of place. Window managers evolved to have titlebars and fancy window management functionality, but the programs remained the same: an inconsistent mess.
|xedit, a text editor||xmessage, an alert dialog|
Unsatisfied with the inconsistent, buggy, and messy implementations of widgets in X-based programs, the Open Source Foundation decided that they would put an end to it at once. They created the Motif widget toolkit, a frontend for X that had a bunch of nice widgets to play with. It had it's own window manager you could use, but didn't have to. The widgets were themeable by the end user, each assigning itself a color based on your current theme, and most importantly, they looked real nice. Motif would persist even through the early Linux days, from its inception in 1988 to it's open-sourcing in 2012. (From a group called the "open source foundation", you'd think that would've been the case from the start...) In fact, it's still alive in some commercial UNIX distributions, like UnixWare! But, as time went on, Motif began to look more and more outdated, programs stopped using it, and the good times started to fade...
|dtfile on Sun Solaris 8||dtfile on SCO UnixWare 7||dtfile on Fedora Linux 36||an example of Motif's heritage and influence|
UNIX was wonderful, and then some random guy in Finland made Linux, which would eventually overtake it in popularity due to the efforts of Red Hat. While this was happening, an open source Photoshop alternative was brewing - the GIMP. The GNU Image Manipulation Program. Those guys had an ace up their sleeves - the GIMP ToolKit, later the GNOME ToolKit, or GTK. GTK was customizable, themeable, simple, and most importantly, usable. GTK never really found much success in UNIX until Oracle decided to shove their awful fork of GNOME 2 into Solaris, which looked like dollar-store Windows XP, which just all around wasn't very good... but over in Linux-land, it was thriving. And you know what? It still is. If you use Linux today, chances are you'll end up using a GTK program at least once per day. GTK and Qt are the current most popular widget sets for UNIX and Linux, and sometimes Windows if you feel real frisky. And that's where we are today.