New feature for Counter Project #83

Description
This issue proposes improving the Counter Project UI and adding a dark/light theme toggle while enhancing accessibility.

Proposed Improvements

• Add dark mode toggle button
• Improve layout using centered card-style- design
• Add smooth UI transitions
• Improve accessibility using ARIA attributes
• Load JavaScript using defer for better performance

Motivation
These updates modernize the interface, improve usability, and enhance accessibility while keeping the original counter functionality unchanged

I would like to work on this feature and submit a pull request.