Add theme-configurable HTML export colors (from PR #387)

- Add optional 'export' section to theme JSON with pageBg, cardBg, infoBg
- If not specified, colors are auto-derived from userMessageBg
- Add export colors to dark.json and light.json
- Update theme-schema.json and TypeBox schema
- Add documentation to docs/theme.md
- Add margin-top back to tool-output for spacing between header and content

packages/coding-agent/src/modes/interactive/theme/light.json

@@ -75,5 +75,10 @@
 		"thinkingXhigh": "#8b008b",
 
 		"bashMode": "green"
+	},
+	"export": {
+		"pageBg": "#f8f8f8",
+		"cardBg": "#ffffff",
+		"infoBg": "#fffae6"
 	}
 }