Windows UI has always been a shambles. Each version of Windows, Office and VisualSomething has at least one new toolkit. It might even look the same as the other ones that came before it, but internally is a different library with different incompatible APIs, bitmaps, workings, etc. Third-Party tools usually also provide their own toolkits that are sometimes made to look Windowsy, sometimes not.

In some cases, APIs wouldn't be too much of a hindrance, and one could replace one toolkit by a later version with updated designs. But the designs themselves also do have a lot of compatibility problems: It is uncommon in the Windows GUI world to use layout managers (similar to CSS flexboxes and the like) that automatically adapt to different window and UI element sizes. Instead, most widgets are positioned pixel-wise. That is also why most dialogs are non-resizeable and things break e.g. when translated strings have different lengths. So a new design which changes sizes breaks all the pixelwise layouts everywhere.

tl;dr: Windows GUIs and toolkits are usually not designed with compatibility and resizeability in mind and break if you change the looks of anything (beyond just changing colors).