Hey!
On that website I see "@media only screen and (max-device-width: 480px) {}" as a text on the site. You should probably put <style></style> tags around that css if you want that to work.
Now to your question:
In "Lay Options" -> "Custom CSS & HTML" there are two CSS fields:
"Custom CSS for desktop" and "Custom CSS for mobile". If you want some css to apply to both mobile and desktop you need to put your css into the two fields.
Maybe it's easier if you just put a style tag into the field "Custom <head> content" (which is below "Custom CSS for Mobile Version"):
<style>
/*your styles for mobile and desktop here*/
</style>
