Nah, the Windows ecosystem never even got close to being consistent.

MS Office had its own UI toolkit and routinely invented new UI paradigms that weren't exposed in any Windows API, leaving people who wanted to look native scrambling to reimplement. This was particularly the case for toolbars. MS Office first invented the so-called "coolbar" and then the ribbon. Internet Explorer also rolled its own toolbar styles in ways not supported in the base Windows API e.g. toolbars with large icons and sliding sub-sections. Inventing custom toolbars was practically a sport on Windows; Netscape also did it.

At the time the most popular media players were WinAmp (totally custom and themeable to boot), RealPlayer (custom UI https://andrewnile.co.uk/blog/remembering-realplayer/), Quicktime (custom UI) and Windows Media Player (mostly but not entirely native).

Even the base utilities that came with Windows weren't consistent with each other. It wasn't uncommon in the Win 9x era to find programs still using Win3.1 style file dialogs ... a few are still buried in Windows today!

The problem got worse when you examined the artwork. The stock icon library in Windows was anemic, so dev platforms frequently had to expand the core library with their own. Delphi apps could be easily identified by the distinctive icons in their buttons (https://zarko-gajic.iz.hr/wp/wp-content/uploads/2017/02/delp...).

Restyling window decorations was also very common. Microsoft themselves did it routinely, for example their flagship Encarta encyclopedia app had totally custom widgets and window styling: https://winworldpc.com/product/encarta/1999

To get online most users were running something like CompuServe (custom web-style main UI https://thedayintech.wordpress.com/wp-content/uploads/2013/0...), or AOL (custom UI https://www.reddit.com/r/nostalgia/comments/ehxb1g/the_aol_h...), or MSN (custom UI https://cdn.prod.website-files.com/6179a66d5f9cc70024c61878/...)

Windows apps of this era were much like web apps are today: they shared some common code for things like rendering menus, buttons or widgets in their settings screens, but the main UI users interacted with were almost always custom widgets that were extremely varied between apps. Win32 was nearly impossible to style compared to HTML so this represented a large investment of developer time, but a custom branded UI was believed to be worth nearly any cost. This is something fundamental to how humans work and is pointless to fight, a lesson the web platform fully embraced giving it an advantage over other UI toolkits of the era.