How to Change theme for AMP mode

@webserviceraj There are plans in place to allow users to select another theme to be used for AMP URLs only, which may be suitable in your case:
https://github.com/ampproject/amp-wp/issues/4478

In the meantime as you’re using reader mode users will be directed to your AMP site only on mobile anyway, with search engines determining the version of your site to be served.

If you move to a confirmed compatible theme, such as Twenty Twenty with transitional or standard enabled your AMP URLs look the same as your non AMP.

Looking at your site you may want to checkout the Newspack theme, which is fully AMP compatible.

Hi James. The Newspack plugin you have suggested works for fine and optimized websites very well.

1. The theme also activated for desktop.
2. Can I use this theme for the mobile version alone?
3. While using Standard mode /amp in URL is ignored will it affect SEO of site?

@webserviceraj We don’t provide support for switching themes for mobile only, but as mentioned there are plans to integrate compatible themes for AMP URLs going forward.

3. While using Standard mode /amp in URL is ignored will it affect SEO of site?

While AMP alone isn’t a ranking factor site performance is. If you use AMP in standard mode you site is AMP by default, without any /amp query var. You can then avail of many of the out of the box performance boosts that can be achieved.

Why don’t you compare your AMP and non AMP site using web.dev while in transitional mode. Bear in mind when served from AMP cache your site should benefit from further improvements.