Add Cardstock theme

[cassidoo]

I am submitting a new Community Theme

Repo URL

Link to my theme: https://github.com/cassidoo/cardstock

Theme checklist

• My repo contains all required files (please do not add them to this obsidian-releases repo)
  • obsidian.css
  • The screenshot file
• I have indicated which modes (dark, light, or both) are compatible with my theme
• I have added a license in the LICENSE file
• If my repo is not using the master branch, please indicate in community-themes.json with "branch": "main".

[ericaxu]

Thanks, looks amazing!