I’ve been searching for a way to create a wrapper page within WordPress without having to install a plugin. I browsed the WordPress files and found wp-blog-header.php in the root install directory. It’s really simple.
I’m not sure when this was implemented. All of the articles I found online mentions plugins that had to be installed or creating a new page within the WordPress admin GUI then include a file, etc. I just need to create a custom PHP file that will display the header, sidebar, and footer.
Depending on the theme you are using will determine additional code between each section (header, sidebar, footer). So check out the page.php or index.php of the theme you are using to see how it is structured.