Added some styling to the nav bar

[aksy2512]

Purpose

The purpose of this Pull Request is to fix #187

Goals

Approach

I added linear gradient and a good hover effect in CSS.

Screenshots

Preview Link

https://pr-188-CALCULATOR-WEBSITE.surge.sh/

Checklist

Related PRs

## Test environment ## Learning

[aksy2512]

@YoshithaRathnayake I have done some changes please have a look

[YoshithaRathnayake]

So sorry for the late request bcs I have examinations in our school in these days. @aksy2512