That is very rose-tinted view of the era. In reality in early 00s lots of software had their own wacky UI toolkits. MS Office is of course the most notable example, but also iirc all of Adobe/Macromedia or every 3d modeling (Lightwave, Maya etc) and audio production software. In the enterprise realm people were doing Java AWT (and later Swing) UIs. And then there were the classics like WinAmp with its iconic theme support, or Mozilla with XUL (and themes).