fix(Header): Adding dark and contrast styles for Header

[bcalvery]

Adding Dark and Contrast theme variables for Header

[codecov]

Codecov Report

Merging #427 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #427   +/-   ##
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  Coverage   88.96%   88.96%           
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  Files          41       41           
  Lines        1387     1387           
  Branches      202      177   -25     
=======================================
  Hits         1234     1234           
  Misses        149      149           
  Partials        4        4

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[bcalvery]

@mnajdova, you worked on the Theme Switcher, could you possibly shed some light on this?

[mnajdova]

@bcalvery when the theme is changed, it is changed for all page. That means, every stardust component we are using on the page, is going to change it's appearance. The headers here, seems to be implemented using the Stardust's Header components, so we will need on changing the theme, to maybe change the docs's background color (the same is already done for the examples (you can see that on dark theme, the background is dark). Another approach would be to wrap these two Header components with Provider and always provide the team's default theme, so it would be the last theme applied. Ping me if you need some help with this.

[codepretty]

Looks good. Is there an issue creating to fix the theme switcher?

[codepretty]

Should this be siteVars.black?

[mnajdova]

@bcalvery I commited the changes required for the docs' Header components to always be black. Ping me if there is anything else.