UIs are presumably more consistent/less varying than documents, so special cases and cascade overlaps should be less of a thing?
You need a different styling inside a specific visual component? You need to fight specificity and override a parent ir a grandparent or a global style.
In most UI systems this is not a concern.
Global is going to mean global, regardless of whatever's managing the appearance, and the only UI systems I can imagine that wouldn't have a problem like this would be those with an entirely uncustomizable global UI presentation.
And for differing stylings inside a specific visual component -- generally, specificity is on your side here, most of the time the selectors involved in component-specific rules will naturally override global styles and you'd only fighting specificity if there's another component specific rule. If that's the case, though, what system would save you from conflicting rules applied to a specific component?
That's only a concern if your UI elements depend on global styles, right?
Meanwhile, UI toolkits such as Qt use CSS (actually, Qt Style Sheets) to style UIs
No.
div {
padding: 15em;
}
This statement makes it sound like every Qt application uses CSS. That's not true. CSS is offered as a way to apply corporate branding to an application. I'm not aware of any FOSS Qt applications using CSS.
Also, if you need to apply a different style, you can just include your custom stylesheet after the global or base stylesheet. As long as you use either the same selectors or more specific selectors, you will be able to override the previous styles. This is assuming the previous developer didn't make the bad design decision of using !important which usually can't be overwritten.