Very troubled by missing H1 tag for titles- Bad for SEO

Everything I know about SEO requires using an H1 tag for page and post titles. Far too late into the process, I have discovered that Hemingway does NOT fit that convention. Very troubling. This is basic SEO. Is there any way to bring that “up to code” without major surgery as I am not using a Child theme and to monkey with the php would be dangerous.