[FEATURE]: Semantic Chat Message Colors

Feature hasn't been suggested before.

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Describe the enhancement you want to request

I would like better chat color customization in the OpenCode desktop GUI.

Currently, user messages, progress updates, tool output, and the final assistant response do not have sufficiently separate theme colors. This makes it difficult to quickly identify where a new instruction begins and where the actual final answer is shown.

Desired behavior:

• User messages should have their own theme color.
• The final assistant answer should have a different theme color.
• Progress updates such as “I’m checking...”, tool output, and status messages should not be styled like the final answer.
• The final answer should be detected using the completed assistant message and its last text part, for example when finish: "stop".
• The colors should provide sufficient contrast for white text.

Technical suggestion:

• Add a dedicated attribute for user messages, such as data-user-message.
• Add a dedicated attribute for the final assistant answer, such as data-final-answer="true".
• Add separate theme tokens, for example:
  • message-user-background
  • message-assistant-final-background
• Detect the final answer in the UI code based on the message state instead of relying only on a fragile CSS selector such as :last-child.
• Support loading custom GUI themes from a user theme directory or from opencode.json.
• Custom themes should remain available after OpenCode updates without requiring users to modify the bundled app.asar file.

Example:

{
  "messageUserBackground": "#254b32",
  "messageAssistantFinalBackground": "#3d477a"
}