Why is the dark theme in my shared CloudWatch dashboard not working?

The issue you're experiencing with the dark theme not working on your shared CloudWatch dashboard is likely not related to a configuration problem on your end. When you share CloudWatch dashboards publicly or with specific users, they are designed to be view-only with limited interactivity.

The HTTP 403 errors you're seeing in your web console when attempting to load the dark theme suggest permission issues. Shared CloudWatch dashboards have specific limitations by design - they're primarily meant for displaying information rather than providing the full interactive experience of the regular CloudWatch console.

Publicly shared dashboards don't support all the interactive features of the regular CloudWatch console, including theme changes. This is a fundamental aspect of how dashboard sharing works in CloudWatch, not something that can be fixed with additional permissions or configuration changes.

If the theme issue is critical for your use case, you might need to consider alternative approaches, such as sharing the dashboard with specific AWS users who have proper credentials rather than using the public sharing feature.
Sources
How do you show input variables on publicly shared cloudwatch dashboard | AWS re:Post
Sharing CloudWatch dashboards - Amazon CloudWatch