Setting CSS color-scheme: light on message iframe

[djanhjo]

Forcing color-scheme: light on the iframe since we don't currently support dark/light mode.

Proper support should come soon.

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Note

• Add color-scheme: light to .gist-message iframe styles in src/templates/message.js to force light theme rendering until dark mode is supported

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[djanhjo]

Closing this since after further testing, this isn't explicitly required. I also did some analytics and releasing this would also impact 11+ customers who have added dark mode support themselves to their in-app messages.