SUI items should be more transparent

### Windows Terminal version

`main` @ 0319ece2192129e74edf27afb6bf8b25b157d111

### Windows build number

N/A

### Other Software

N/A

### Steps to reproduce

1. Open the settings UI
2. Navigate to the Startup page
3. Look at the "Launch size" or "Launch parameters" expander

### Expected Behavior

The Windows Community Toolkit Gallery shows what `SettingsExpander` and `SettingsCard` looks like. This should be the styling on it:

No hovering:

Hover over header:

### Actual Behavior

In the SUI, which uses the ported versions of `SettingsExpander` and `SettingsCard`, the color/transparency looks slightly different:

No hovering:

Hover over header:

I believe this has to do with the background material used in the settings UI. Like, I think we're not using mica and the WCT gallery is, or something like that.