Improvements to the admonition classes

[choldgraf]

In #183 we identified a few potential improvements to make to the admonitions, here are the main points from @jorisvandenbossche:

Color choices

I actually was thinking of the success/danger/warning/info colors, but I suppose those are not available with a -light variant? (but could maybe also use a function to lighten it: stackoverflow.com/questions/1625681/dynamically-change-color-to-lighter-or-darker-by-percentage-css-javascript)

User variable over-rides

It would be nice to use variables here in a way they are still overrideable by theme users (but that's probably covered by #138. I also don't know if you can use css variables in sass and have them preserved in the css output)

Custom styling with sphinx-panels

We are adding some custom styling here (like the shadow), while some of those things are also covered by bootstrap classes. I am wondering if we can't make more use of that (like the sphinx-panels). Althought that then goes back somewhat to how it was before .. as that would need a custom translator to add those classes to the admonition node

[hoetmaaiers]

User variable over-rides

This is implemented in #190. Feedback is welcome.

The lighter color is now explicitly definable.

[jpmckinney]

Can be closed?

[choldgraf]

Yep