[devtool] fix scrollbar styling (#81814)
* The scrollbar styling was lost before while we enabling the draggable panel. Share that styles between all scrollable content
* Migrate the inline style to css and dynamic css into css variables, so we'll create less dynamic object during rendering.
<video src="https://github.com/user-attachments/assets/df63169f-4846-4c53-91b4-3ca0585dd76d" width="400">
