Support multiple message elements in Gmail dark theme

[timche]

Refactor the Gmail message dark theme handler to support multiple message elements simultaneously instead of tracking only a single element.

Summary

The dark theme message handler previously tracked a single message element and would revert its theme when a different element was encountered. This change updates it to maintain dark theme controllers for multiple message elements, enabling proper theming when multiple messages are visible in the DOM.

Key Changes

• Changed from single themedElement and controller variables to a Map<HTMLElement, DarkThemeController> for tracking multiple elements
• Updated selector from $() (single element) to $$() (multiple elements)
• Implemented cleanup logic that reverts themes for removed elements and applies themes to new elements
• Maintains a Set of current message elements to efficiently detect which controllers should be cleaned up

Implementation Details

• The function now iterates through existing controllers and removes those whose elements are no longer in the DOM
• For each current message element, it checks if a controller already exists before creating a new one, avoiding duplicate theme applications
• The dark theme configuration remains unchanged; only the lifecycle management was refactored

https://claude.ai/code/session_01Q3HANa7VjTStHLgCT1USHa