When I want to say that a website has a main header, I and almost everyone else say the main site header, but I feel this is wrong because the adjective refers to the header, not the website. The same goes for the main site style. It means that a website has a style used across all pages, but the way it's written, it seems that we're referring to the style of the main website.
Here's an example in case you're not familiar with web development terms https://wordpress.stackexchange.com/questions/343300/how-to-disable-primary-main-site-header-and-logo-in-wordpress-astra-theme