fix(button): add initial styles for button

[joneff]

Related to #798

• sass variables allow us to have null value and do not generate output when there is no need, which is not the case with css variables;
• on top of that, css variables, if not provided with fallback and have null value will fallback to prop: initial
  • focus state, per se, is null, so should we provide a fallback to normal state?
• should we have a sort of handling of css variables, so we use either css or sass variables?
  • prop: k-var( --kendo-css-var, $kendo-sass-var ) and have a $use-css-variables switch...

Apart from that, it looks like it would be doable to have e theme that's based on css variables but can also benefit from sass toolchain.

[joneff]

Merging to have a patch version of the theme, so we have something deployed to test against.