Styled and add function on Calculator

[onealanil]

Changes Made:

• Improved the CSS to enhance the visual appearance of the calculator, giving it a 3D look etc.
• Added functionality in JavaScript to delete the last entered digit.

Type of Change:

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (improvements to existing features)
• Refactor (code improvements without changing functionality)
• Other (please specify)

Screenshots (if applicable):

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings or errors
• I have tested my changes thoroughly and can confirm they work as expected