Teal primary button fails accessibility contrast check

[hartsick]

In setting up accessibility specs on Colorado, I ran into a contrast-related failure on primary buttons in teal (.button--primary). Currently the buttons look like this (and have a ratio of 3.44):

The middle button is the one failing our contrast tests at the moment, though suspect at least the smaller button is, as well.

Here's the page that describes the failure we experienced, and has some tools to try things out to improve the contrast: https://dequeuniversity.com/rules/axe/2.3/color-contrast?application=axeAPI

[hartsick]

All merged in! Here's the relevant PR: #19