For your specific theme/site, the most straightforward way to accomplish this is by adding the following to your CSS:
margin: 0 0 0 1rem; /* This will affect the content area */
margin: 0 1rem 0 0; /* This will affect the sidebar */
This won't have any immediate effects, because it will be equivalent to the current setting. Increasing the last number of the "#primary" rule will increase the content area's left margin, pushing it inward.
Increasing the second number of the "#secondary" rule will increase the sidebar's right margin, to the same effect.
As long as you keep them equal, they'll stay an equal distance from the outside of their parent element - which, in this case means they'll stay an equal distance from the outside of the page.