Improve color contrast

[mgalbritton]

When in dark mode, the text contrast makes the text almost unreadable.

OS version

Windows 10; Simplenote 1.1.6

Reported at #1351437-zen

[davewhitley]

Developers: we'll need to evaluate all the text contrast and see if it meets AA.

[mirka]

Some improvements we should make:

• Light theme
  • Change the $blue from #4895D9 to #287AC3
  • Change $gray from #899199 to #6F7780
• Dark theme
  • Change the background from #2D3034 to #282A2E
• Toggle switches: Add cues to that don't rely on color (currently hard to differentiate with red–green color blindness)

[codebykat]

There were lots of color updates in #1565 which have hopefully addressed the worst of the contrast concerns, particularly in dark mode. I think it's on the radar to do a more comprehensive contrast audit, but for now I'm closing this issue since it's been almost a year since the last report and quite a bit has changed. Please do re-open if there are specific problem areas still.