What really should happen here is that the built-in styles shouldn't allow changing any items that don't actually affect the drawing. But this is fairly annoying to track down.

Changing width in #def-draw-error style has no effect.

Found that similar issue happen with #def-selected line style (only color changing has effect for this style), but it reuse width and stipple type from #def-active-grp instead.

That's intended. The bug here is that the built-in style lets you change this parameter.

The bug here is that the built-in style lets you change this parameter.

Sometime, changing built-in styles are really needed.

For example, if I want set "red" background, default built-in "red" highlighting for selected entities is mostly useless.

User should has possibility to tune default UI & styles according personal preferences

Sometime, changing built-in styles are really needed.

You misunderstood. Being able to change e.g. selected entity color is useful and intended. The problem is that although the #def-selected style exists only to allow you to change selected entity color, it shows many more changeable parameters that do not affect anything. These parameters should not be shown.