style: 🎨 update theme setting to align with system default

Author: assafdori

opencode/opencode.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://opencode.ai/config.json",
-  "theme": "opencode",
+  "theme": "system",
   "model": "github-copilot/gpt-4o",
   "autoupdate": true
 }