MNTOR-5282 - Add email dark mode per guide

[codemist]

References:

Jira: MNTOR-5282
Figma:

Description

QA identified that the new work-arounds introduced more negative changes. I reviewed a few pieces of documentation
https://www.emailonacid.com/blog/article/email-development/advanced-mjml-coding/
https://www.litmus.com/blog/the-ultimate-guide-to-dark-mode-for-email-marketers
to guide me on adding dark mode support, e.g. swap to white-text variants, and the hero/card/footer/body backgrounds darken so the swapped logos and body text remain readable instead of sitting on light-colored "fields."

How to test

We can't really test this until it's on stage.

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• If this PR implements a feature flag or experimentation, I've checked that it still works with the flag both on, and with the flag off.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[codemist]

The hero section was still #F5EAFF (light purple), the footer wrapper was #F9F9FA (light gray), the body was white. So when the swap fired, the new white-text dark logo was sitting on a light-colored section, readable on Storybook (where you see the whole email), but on a real client it looks like a white logo on a bright box. That's the "purple field under the Monitor logo" QA reported.

[joeherm]

Looks good and appreciate the technique references listed. Not 100% sure on the testing side (can leave that to QA since it's client dependent), but code looks good.