Refactor theme detection logic for cleaner DOM execution

[Ayush1519]

This PR simplifies the theme-switching logic by:

• Wrapping the script inside a DOMContentLoaded event
• Using a boolean flag (useDark) for readability
• Replacing verbose if...else with classList.toggle() for clarity

Improves maintainability, reduces repetition, and ensures reliable DOM access timing.

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[deeleeramone]

Hi, thanks for the PR.

Can you tell me how you have tested this code and under what conditions? Which DOM execution are you referring to, specifically?