fix: use official CMU brand color - #6D6E71

[natsuki615]

• match the iron gray color on https://brand.cmu.edu/visual-identity/colors
• change green to blue in build status so it's colorblind friendly, also adhering to official cmu colors
• note: cmu.css stays the same only with rgb(0, 0, 0, .6) swapped out for #6D6E71, but I beautified it before the change then minified it again for commit, hence the whole file looks changed.