adjust site to screen size (newbie)

Using Coraline theme with full screen background. How do I set it so it looks the same on all screen sizes. I am building the site using a large monitor but when I view on laptop or other screens the text is covered by the image on the bottom of the screen. Is there a way to set the site to zoom or resize according to screen size?
site is https://thebrownchurch.org/wordpress-sample/

Separate question how do I check it and what do I do if it doesn’t look right on different browsers IE Chrome Safari Firefox Opera?

Any assistance would be greatly appreciated.