Reduce page width on whole website

I have the Sparkling theme and I want to reduce the width of the post and sidebar - What code can I paste in to do this? (The header should reduce in line with the rest of the page)

I still want the site width to be dynamic though - for example if viewing on a phone, i dont want the page width to be tiny, I want it to fill the screen.

Thanks very much for your help!

Hey there

I hope you are well today and thank you for your question.

You can try achieving this by adding the following CSS code in the Custom CSS option of your theme on the below path.
You can adjust the percentages to your liking.

Admin Area -> Appearance -> Customize -> Additional CSS

/*reduce sidebar increase main section */
@media (min-width: 992px){
    width: 25.333333%;
    width: 72.666667%;
  img.single-featured.wp-post-image {
    margin: 0px auto;
    width: 100%;

Best Regards,