chore(PPDSC-2136): add accessibility tests for newskit dark theme

[evgenitsn]

PPDSC-2136

What

1. Background - we have to test the dark theme for accessibility (colour contrast)
2. What did you do - added a way of running the a11y tests in both light and dark theme.
3. What does the reviewers should expect - running the e2e tests, the a11y set of tests should include both light and dark theme and all tests should pass.

How to test:

Run locally yarn e2e:build && yarn e2e:serve and then in a separate terminal run yarn e2e:comps and start the a11y set of tests, or check if the CI e2e tests are ok.

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

Before:

After:

Who should review this PR:

[newskit-engineering]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2136-add-accessibility-tests-for-newskit-dark
Storybook: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2136-add-accessibility-tests-for-newskit-dark/storybook