Textbox blends into background in Agents App Light+ theme

Light+:
<img width="521" height="212" alt="Image" src="https://github.com/user-attachments/assets/544d919c-6da3-4b5c-89ee-60422e66bbe2" />


Compare with Light 2026:
<img width="521" height="181" alt="Image" src="https://github.com/user-attachments/assets/d94f075c-faeb-4287-9ba9-493375597e0c" />