That's just plain FUD. The only release I remember breaking CSS was 3.20, and that was because all the CSS was refactored to allow for way more theming possibilities.
Regardless of that I don't see anything based on "cascading styles" as lending itself to a stable theming system anyway. The CSS is probably going to change any time a widget is added/refactored/bugfixed, this holds true on the web as well once you build up a complex library of React components or whatever. The point of it is that there are multiple styles from multiple sources that can cascade together. It's powerful but it can result in a lot of complexity, anyone who's had to add !important directives can attest to that. It needs to be strictly managed by the developers to really work correctly and to prevent the style overrides from getting out of hand. Allowing custom user CSS is only for power users who understand the caveats.
Regardless of that I don't see anything based on "cascading styles" as lending itself to a stable theming system anyway. The CSS is probably going to change any time a widget is added/refactored/bugfixed, this holds true on the web as well once you build up a complex library of React components or whatever. The point of it is that there are multiple styles from multiple sources that can cascade together. It's powerful but it can result in a lot of complexity, anyone who's had to add !important directives can attest to that. It needs to be strictly managed by the developers to really work correctly and to prevent the style overrides from getting out of hand. Allowing custom user CSS is only for power users who understand the caveats.