Ah – sorry, I completely misunderstood!
You can hide the header & menu sections on specific pages using this CSS syntax:
.page-id-25 .site-header, .page-id-30 .site-header {
display: none;
}
Replace the page IDs with your actual pages – you can grab the numbers from the browser’s address bar when you’re editing the page, or in the body tag when you’re viewing the browser source.
Let me know if this is more along the lines of what you wanted!
Also a heads-up, don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.