Adjusting content area right side

Hi, I have the sidebar sticky to the left but the main content is now very wide out to the right (making the screen area wide to make the sidebar stick to the left). Is there an easy was I can get the main content more narrow? Probably to bring in the right margin? Everything displays just fine on a mobile so I don’t need to change that, just a desktop displays far too wide.
Thanks for any ideas. Justin

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