[articles] skeleton loading cards dark mode

Summary

• Skeleton loader has been updated for the different variants for article cards, in both light and dark mode.

• As an extra, colors for the dark mode in the count in the header for the articles count has been fixed.

Checklist

• I checked my UI changes against the design and there are no notable differences
• I checked my UI changes for any responsiveness issues
• I checked my (code) changes for obvious issues, debug statements and commented code
• I opened a corresponding card on Clickup for any remaining TODOs in my code
• I added a short description on how to test this PR (if necessary)
• I added a storybook entry for the component that was added (if necessary)
• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged