The themed UIs of that era were very superficial -- if they applied to serious software at all, they were just a cosmetic layer on top of an otherwise well-engineered interface, and could be easily disabled. Most people I knew, for example, disabled the theming engine that shipped with Windows XP. Most applications that supported UI skinning still had a default or fallback UI that adhered well enough to modern conventions.

Not so much anymore. The abandonment of any coherent organizing principle to UI layout in favor of pure aesthetics has been a massive regression. Reasonably complex software often doesn't even include menu bars anymore, for example.