Mario Bras

Rename concepts to `standard_case`

61

Following the changes introduced by [P1754R1](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1754r1.pdf) to the concepts library in C++20, consider renaming all concepts to `standard_case`, for consistency.

Support setting local CSS file in style global directive

2

Right now you can define a CSS style as a global directive to extend a base theme. It appears this can only be written inline, e.g. from documentation: ```markdown ---...