Fix #1708: Update Rewards Panel Icon to have more contrast in dark mode

[kylehickinson]

Summary of Changes

This pull request fixes issue #1708

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

• Enable Rewards
• Go into dark mode
• Visit a verified publisher, icon should be easier to see (white checkmark)

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).

[iccub]

Does it look good in light mode on iOS 12 too?

[kylehickinson]

Yep!

[Brandon-T]

Hmm.. I was looking for the code where this gets dynamically updated when you switch modes but couldn't find it.

[kylehickinson]

@Brandon-T It doesn't get updated based on light/dark mode. We are just using the one that has a white checkmark regardless