The reason desktops were customizable in the 90s is that they followed strict GUI rules and used the same toolkits. Nowadays, apps want to have control of their

The reason desktops were customizable in the 90s is that they followed strict GUI rules and used the same toolkits. Nowadays, apps want to have control of their UI to a higher degree, which makes it difficult to provide customization. The same applies to the web: since every app wants to have it own UI, there is no space for user-level customization.