feat: Update readme with theme description

[chrisdondigital]

DCMAW-18949: Update README to align with design systems

• Update readme with theme description

Evidence of the change

Checklist

Before creating the pull request

• Commit messages that conform to conventional commit messages.
• Ran the app locally ensuring it builds.
• Tests pass locally.
• Pull request has a clear title with a short description about the feature or update.
• Created a draft pull request if it's not ready for review.

Before the CODEOWNERS review the pull request

• Complete all Acceptance Criteria within Jira ticket.
• Self-review code.
• Successfully run changes on a testing device.
• Complete automated Testing:
  • Unit Tests.
  • Integration Tests.
  • Instrumentation / Emulator Tests.
• Review Accessibility considerations.
• Handle PR comments.

Before merging the pull request

• Sonar cloud report passes inspections for your PR.
• Resolve all comments.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud