Expected behavior

Specifying "skin-sidenav" should allow be to overwrite the skin color for the sidenav

Actual behavior

I had to look through the source code to find the sidenav color is being dictated by the "skin-flat" variable. That is not straightforward at all. Why is flat = sidenav background color? Wouldn't using a variable like "skin-sidenav" make more sense?

This kind of really brings me to the following feedback as I've really enjoyed this product the past few years.. I'd really like to see this system removed and the _custom.scss files reintroduced. The newly introduced "skin" architecture:

1. Is a more restrictive version of a system people already know (scss/css)
2. Is a huge pain point for people to upgrade
3. Makes it harder for people to adopt your product as they now have to relearn what/where/which "skin" variables impact the styling
4. Is harder to identify what variables impacts what styling, rather than a well known established existing system (scss/css).
5. Makes future releases more risky as you may change which variables impact which styling.
6. Doesn't introduce any significant advantages over scss/css
7. Will cost you more in support/answering questions/documentation
8. Decreases the overall developer experience using your product

I can't honestly think of a solid pro here for this "skin" architecture that isn't already addressed in a better, more flexible, and existing system (scss/css). What was the thought process behind this that we don't already solve with scss/css in a less clunky manner?

Resources (screenshots, code snippets etc.)