Use the previous "select mode" dark toolbar style for Write mode

Let's try adding an additional affordance to write mode to help you understand what "mode" you're in, writing or design, by leveraging the pre-existing styles for select mode. This way there's a clear visual affordance to indicate the difference in experiences. 

### Visual 

![Image](https://github.com/user-attachments/assets/a2f4d6f5-5b9e-4e6d-b4ef-9a1eb81616d6)

Related #65298. 