Update style.css

[Durgesh4993]

In this CSS code:

• CSS variables are used to define colors, providing a way to easily update the color scheme throughout the code.
• Styles are organized logically, with comments providing context for each section.
• The :root pseudo-class is used to define global variables.
• Selectors are used to target specific elements, classes, and IDs in the HTML markup.
• Transitions and hover effects are applied to buttons for a smoother user experience.