CL - Theme Helper Unit Tests Implemented

[alakkaya]

📋 PR Description

This PR adds comprehensive unit tests for the ThemeHelper class, focusing on validating all functionality including theme initialization, persistence, application, and color management. The tests verify the singleton behavior, theme persistence in UserDefaults, different theme types (light, dark, custom), and proper color handling.

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✅ Checklist

• Code follows the project standards and guidelines.
• Relevant unit tests are written and all tests are passing.
• Test coverage is adequate for the changes.
• Any unnecessary files or debug statements have been removed.
• Documentation is updated where necessary.
• The PR has been reviewed by at least one team member before merging.

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🛠 Steps to Test

1. Run the test suite using ⌘ + U in Xcode or via xcodebuild test.
2. Ensure that all tests pass successfully.
3. Validate that EnvironmentHelper retrieves the correct values based on the current environment configuration.