content: add CSS properties corresponding to our palette
content: add CSS properties corresponding to our palette

At least during the development phase, this will make it easier to
implement consistent styling across the site. If needed, we can later
either revert to constant values, or use a preprocessor.

Change-Id: I94d5987d69f2e251304c9c2907b0892c82c6ac76
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/450849
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite