Add ENV variable allowing to choose default color schema

[tom2drum]

Description and Related Issue(s)

Resolves #1868

Proposed Changes

Added NEXT_PUBLIC_COLOR_THEME_DEFAULT for set up default color theme of the app

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable, I have updated the list of environment variables in the documentation and made the necessary changes to the validator script according to the guide