Overwriting Styles in extra.css is a really bad solution. Destroying old declarations due overwrite is no good webdeveloping at all.
Having two interfaces for the same things to change, is also wierd. How the user should know, wehere to change.
The only good solution is : Give the User One Place to change all.
I also would be lucky, without any Style-Backend and a good collection of CSS Files. For developing this is very handy, instead of having multiple options to change the look.
If you think, that putting all into the Backend, would create an overload, the System is bad, and it there must be found a better way to organize it.
But on the second way, it is my opinion that no HTML rookie should manage a community alone.
How exactly is overwriting in extra.css a bad solution? Its exactly what it is used for, and it gives greater control (Its forward compatible with different versions of XenForo).
There aren't two interfaces for editing; there is the style properties, which is a UI for styling, than there are the stylesheet templates where you can customize the code directly without dealing with pre-existing properties. Its quite logical, and god help Kier if he ever changed it (Which I doubt) as limiting people to just style properties would screw over every designer who wants to design outside the box.
If we were restricted to just style properties everyone would have to work within the same model (For the most part anyways). For example, I don't generally use the default navigation or forum_list properties? The reason being is they're limited for beginners to use and are functional only so much as they'll need to design using them. I make use of most of the navigation section as well as adding a few extra things. This wouldn't be possible with just style properties (Or would require that every designer make their own style properties).
I already explained; the style properties are there for people who do not directly understand CSS. While there is a learning curve, it is much smaller than there is for learning CSS. Kier himself has stated that he expects professional designers to make use of template edits as much as possible. It isn't black and white, there is an in-between that allows people to use both. Having to just have one choice is ridiculous as it would be too advanced to beginners, or too limited for professional designers.
The system isn't bad, there is just a lot of things that are used in XenForo. In the framework I've done I've added 20 (Will be adding a few more) style property groups and 302 style properties. If all of that was in the core (Some of it would fit in other groups and some are to override existing properties) it would definitely overload people. It isn't that the system is bad, it is that the system is extensive. XenForo is built upon a very rich extensible CSS framework that gives a lot of control over everything, the problem is most people will never have to go outside of the style properties. XenForo is balanced for professional designers and for beginners, changing that will piss off one group or the other.
I also think that administrators should have a basic understanding of CSS, HTML, and enough skill to debug their own issues. But we live in the real world, and maybe 20% have the skills stated above (Or at least enough to get by). XenForo is a commercial product, they cannot just pick one group to cater to the beginners or experienced, they have to cater to both which they have done very well.