CI check for diffs containing hex colors against the Material Color System

[PKief]

To pick up the comment of @sndst00m:
#1129 (comment)

[ghost]

Do you have a preferred JavaScript library for tests?

[PKief]

As test framework I'm using mocha for this project. If you are comfortable with jest we could also switch to that.

[ghost]

I prefer Mocha's TDD format. We should ideally limit such a CI to the diff for said pull request or commit