Improves contrast and resolves #76

[amandafrench]

Resolves the 2 remaining issues of 4 in #76 by changing link text on external link to black and using ROR theme dark gray for gray buttons on blog.

[amandafrench]

[amandafrench]

If these changes look good I can make them on ror-app as well

[amandafrench]

This is what the dark gray button would look like on ror-app

[lizkrznarich]

@amandafrench ror-app shares its base css files with ror-site, so let's just merge this to dev and check that ror-app has the changes.