feat: update global css style and add highlight effect for header

src/app/app.component.html

src/app/app.component.scss

src/app/footer/footer.component.scss

src/app/header/header.component.html

src/app/header/header.component.scss

src/assets/scss/_color.scss

src/assets/scss/_icons.scss

src/assets/scss/_layout.scss

src/assets/scss/_shapes.scss

src/styles.scss