Hello, @Anshu,
1- The tax label appears in GST already on your site see the image for a better reference: https://i.is.cc/ky5Ty0x.png
2- Actually, The code you added works well but you just set the width to auto. I suggest you define the width of it like this: https://i.is.cc/kxSdb1Q.png so that you will be able to see the change.
Regarding the CSS there is no need to add it in the theme core files you can simply add the CSS code in the Dashboard >> XSTORE >> Theme Settings >> Theme Custom CSS >> Global >> See the image: https://i.is.cc/ky298Re.png
Regards 8Themes Team.