A Shopify theme is structured very different from full HTML files, which have <html> <head> and <body> tags, so if that’s what you’re doing, it will not be compatible and will most definitely break things.
Shopify pages are structured as a json template file, liquid section files, and rendered liquid snippets, with the Page being the data that gets injected at read time.
So it really depends on what exactly you’re trying to do. Are you adding a new section to an existing page, replacing a full page layout, or building something from scratch?