Refactors theming app - part 3

[fsamapoor]

Summary

Following previous PRs taking advantage of PHP8's constructor property promotion, I have also made the required adjustments to the classes in the theming app.

I decided to split the changes into multiple PRs to make reviewing the changes easier.

The improvements in this PR include:

• Using PHP8's constructor property promotion
• Adding return types
• Removing redundant docblocks

Checklist

• Code is properly formatted
• Sign-off message is added to all commits

[github-actions]

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