PPDSC-2474

What

Added the new themes which are used only for Storybook and just for the Scenarios named "Styling overrides".
The themes storybookNewsKitLight and storybookNewsKitDark are based on the newsKitLight and newsKitDark but with overridden colours.

Usage: If you have a Styling overrides story in your work you have to provide a decorator with <ThemeProvider> and pass the context.name to the createCustomThemeWithBaseThemeSwitch as last argument. And also the theme is applied only if there is a scenarios section called Styling overrides.

You can have a look at the banner.stories.tsx.

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:

How to test: