When it comes to SEO, all a WordPress theme can really do is:
1. Be fast
2. Present your content as high in the source as possible
3. Use schema (which may not even do anything for your SEO)
GP focuses on all 3 of those items.
SEO itself is more about your content than anything else. Has anything else changed? Has your overall content structure stayed the same? Links are all the same? Has anything been reported in your search console?