How can I adjust desktop website borders for mobile devices?

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I am having trouble with the (Body) side margin/borders on my desktop site, because I want to keep them showing on just desktop devices over a certain screen width, but small mobile devices, I want to have the site interface, just normal, but it continues to display the borders even on the small mobile devices. My theme is a custom bought theme, but i'm hoping there is some code to alter this problem?