Feature: Support changing luminosity of Acrylic backdrop

### What feature or improvement do you think would benefit Files?

~~Add an extra option to control whether Files is allowed to use the system Mica as the background layer directly (i.e., use the system title frame directly).~~

~~I am using [DWMBlurGlass](https://github.com/Maplespe/DWMBlurGlass), which works very well in various default WinUI 3 programs, and I hope Files can support it too. Since the principle of DBG is to replace the rendering of system background frame, it can be achieved as long as Files uses system frame.~~

I think it would be nice to allow the luminosity of Acrylic to be modified.
Now it only has Acrylic Base (~50%) & Thin (~90%).

### Requirements

~~- Add a 'System Default' option to the 'Appearance - Background Material' drop-down menu;~~
~~- Modify the logic for the application background so that when the above option is selected, it directly reverts to the system-level background.~~

- Allow changing luminosity of Acrylic backdrop

### Files Version

4.0.11.0

### Windows Version

10.0.26200.6901

### Comments

_No response_