Any website you make it must be seo and cro friendly.
This shouldn't impact anything other than your traditional store metrics.. assuming your new theme is an optimized theme
What do I mean by that? Here's two examples;
You can always benchmark test your historical theme vs your new in regards to performance. Keep in mind time-based tests can have potentially misleading results.
Hopefully, that was helpful!
The answer here is yes. The new theme can effect in a positive and negative way. Best way to check it go to Google analytics and compare the last 30 days data
Compare user avg time on the website
The conversion rate of the website
Avg pages per session of the website
Checkout the speed of the website on mobile and desktop
Yeah a few ways: