[Dock] Add transparency adjustment for acrylic material

### Description of the new feature / enhancement

Could we add an option to adjust the transparency of the acrylic Dock?

### Scenario when this would be used?

Users who want to customize their desktop aesthetics or improve visibility/contrast based on their wallpapers.

### Supporting information

_No response_

The issue names no files, tests, or entry points. First locate the Dock's acrylic implementation and settings entry point, then clarify the supported transparency range and how the adjustment should be verified.

Written by the indexing model from the issue text.