modify text/background colours for appropriate contrast ratios as per WCAG

[VatsalBhuva11]

Fixes #1270

Brief Description:

Some of the buttons did not have a sufficient contrast ratio as per the W3C Web Content Accessibility Guidelines (WCAG). This PR solves that by changing the text/background colours of some of these buttons (Stripe, Foursquare, Google Maps, HERE Maps) buttons under the /api/examples endpoint to ensure that these buttons have a contrast ratio >= 7.

Changes:

1) Stripe:

• Text colour changed from #fff to #000
• Contrast ratio changes from 2.65 to 7.93 2) Foursquare:
• Text colour changed from #fff to #000
• Contrast ratio changes from 2.50 to 8.39 3) Google Maps:
• Text colour changed from #fff to #000
• Background colour changed from #20a360 to #20b067
• Contrast ratio changes from 3.25 to 7.47
• Note: With the original background colour, the maximum contrast ratio is obtained with #000 (6.47). Hence, the background colour is tweaked a bit so that the green colour of the background is not affected a lot while still ensuring a >= 7 contrast ratio. 4) HERE Maps:
• Text colour changed from #0f1621 to #000
• Contrast ratio changed from 15.72 to 18.19
• Although the contrast ratio was high enough initially, the text colour has been changed to black to ensure a uniform text colour throughout all buttons (either black or white)

Screenshots:

References used:

• https://www.w3.org/TR/WCAG21/#contrast-minimum
• https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
• https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
• http://tools.cactusflower.org/analyzer/

[VatsalBhuva11]

Can you fix the issue with File Upload as well?

Sure, I'll do it. I'm very sorry, I must've overlooked it in your original request. I changed Google Maps instead of File Upload. I'll make the necessary changes.

[VatsalBhuva11]

Should I make a new PR, or modify the commit in this PR to add these changes to ensure a single commit?

[VatsalBhuva11]

@YasharF I have made the requested changes in this single commit itself. The text-colours of the buttons that required changes are now either set to #000 or #fff, are uniform, and have a sufficiently high contrast ratio. I also changed the background-colour of the Google Maps button to the one used in the branding doc (#0f9d58). If any more changes are required, do suggest them.

[YasharF]

Thank you