Codebase: Refactored themes

[yaira2]

Resolved / Related Issues

• Switched property in view model and resource factory to string. This accounts for empty strings which indicate the default value. If we didn't use blank strings to indicate default, we'd have to provide a separate value for light and dark mode.
• Added LoadAppResources method to AppThemeResourcesHelper
• Call LoadAppResources from the ViewModel instead of the View.

Validation
How did you test these changes?

• Built and ran the app
• Tested the changes for accessibility

Screenshots (optional)
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