Start by adding placeholder values (like zero) to theme.json for contentSize and wideSize under settings > layout. This unlocks the layout interface in the site editor even though the values aren’t final.
Use the site editor’s styles panel to set the actual values: 520 pixels for content width and 750 pixels for wide width, matching the original Bosco theme measurements.
Test your settings by editing a post – you should see the content properly constrained to 520 pixels, and images should offer a wide alignment option that displays at 750 pixels.
Finally, use the Create Block Theme plugin to export your styles changes back to theme.json, making these settings permanent in your theme files rather than stored in the database.