Find My Mouse: Handle default color for brand new settings (#42182)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
Give a default color transparency as before instead of leave background
black and foreground white.

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## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
1. Delete setting for find my mouse
2. Start find my mouse, the backdrop and foreground are semi-transparent
3. Start settings, and the default settings should persist correctly.
4. Start powertoys again, make sure the default color is still
semi-transparent
5. Other settings change can be handled and persisted correctly

---------

Co-authored-by: Niels Laute <niels.laute@live.nl>

src/settings-ui/Settings.UI/ViewModels/MouseUtilsViewModel.cs

@@ -50,10 +50,10 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
             _findMyMouseDoNotActivateOnGameMode = FindMyMouseSettingsConfig.Properties.DoNotActivateOnGameMode.Value;
 
             string backgroundColor = FindMyMouseSettingsConfig.Properties.BackgroundColor.Value;
-            _findMyMouseBackgroundColor = !string.IsNullOrEmpty(backgroundColor) ? backgroundColor : "#000000";
+            _findMyMouseBackgroundColor = !string.IsNullOrEmpty(backgroundColor) ? backgroundColor : "#80000000";
 
             string spotlightColor = FindMyMouseSettingsConfig.Properties.SpotlightColor.Value;
-            _findMyMouseSpotlightColor = !string.IsNullOrEmpty(spotlightColor) ? spotlightColor : "#FFFFFF";
+            _findMyMouseSpotlightColor = !string.IsNullOrEmpty(spotlightColor) ? spotlightColor : "#80FFFFFF";
 
             _findMyMouseSpotlightRadius = FindMyMouseSettingsConfig.Properties.SpotlightRadius.Value;
             _findMyMouseAnimationDurationMs = FindMyMouseSettingsConfig.Properties.AnimationDurationMs.Value;