Update button colors for greater contrast (#1094)

[bruceplai]

Closes #1094

• Update default button colors in theme

• Remove button color style from Collaborate with Us page

[codeclimate[bot]]

Code Climate has analyzed commit 4021fd06 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 80.9% (0.0% change).

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