Mac OS had themes (briefly) back in 1998 [1]. Linux (GTK+ and Qt) still do. Windows has supported custom fonts/colors since at least 1992, even if the result wasn't always tasteful [2].

[1]: https://www.blakespot.com/mac/Images/Gallerypix/drawing2.jpg [2]: https://blog.codinghorror.com/a-tribute-to-the-windows-31-ho...