reduce the space on right and left sides of the pages and posts

Hi there,

do you want to create full width pages so you content goes from edge to edge of the browser?
Or do you want change the body background color so the entire page is white ?

No, I want to reduce the margin on both sides of the pages and posts.

You can increase the Container Width in Customizer > Layout > Container.

If you want to change the spacing around the content then in that same customizer setting you can change the padding.

Thank you

You're welcome