Add proper dark mode

[xixixao]

🌈 Feature

The editor is already mostly dark, but the output window (iframe) flashes white (easily fixable), and errors also have white background (harder to fix). Please consider adding a full dark mode, so that the screen isn't shining bright white in the output area.

[xixixao]

Still my #1 gripe.

[github-actions]

This issue has automatically been marked stale because there has been no activity in a while. Please leave a comment if the issue has not been resolved, or if it is not stale for any other reason. After 2 weeks, this issue will automatically be closed, unless a comment is made or the stale label is removed.

[github-actions]

This issue has been automatically closed because there wasn't any activity after the previous notice or the stale label wasn't removed.