Brad Frost, of atomic design fame, wrote a history of themeable UIs as part of a deep dive into design tokens. He writes, “Design tokens may be the latest incarnation, but software creators have been creating themeable user interfaces for quite a long time!”

About Mario and Luigi from Super Mario Bros.:

It’s wild that two of the most iconic characters in the history of pop culture — red-clad Mario and green-clad Luigi — are themeable UI elements born from pragmatic ingenuity to overcome technological challenges. Freaking amazing.