Using the same theme for the page that shows in the popup is rarely a good idea. Usually, you don’t want to show elements like the menu, the page title, the slogan, the footer etc. on those pages.
If you use a framework like Foundry
or alike you can set up the pages you want to show in the popups with just the elements you want. If you use a ‘regular’ theme, the easiest and most flexible way is to use a blank theme for those pages. You can style such a page with the options in stacks. There are several themes like that available, but personally I prefer Blank 5.1 by ThemeFlood
, because it has the most styling options.
Last resort would be to use custom CSS to hide the elements you don’t want. I do not recommend that unless you are familiar with CSS.