change to acceptable contrast colors (fixes #925)

[GaurangTandon]

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

@catarak I have verified that the colors that I have now selected are well above the 4.5AA guideline. Is the new scheme fine?

One question though: secondary-text-color and inactive-text-color are now both the same. Would you want me to change them slightly? Secondary text color could be a bit bolder perhaps, or just simply an even darker shade of grey? Something like hsl(0, 0, 10%)?

[catarak]

can you change these colors based on the design on zeplin?