Center Inner pages

Hi, I’ve read a few other posts with the same issue but can’t find an ‘idiot’s guide’ to fixing it. I’d like to align the inner pages the same as the main Catalogue page so it doesn’t align top left.

Would you be kind enough to let me know exactly what code I need to add to the Inner Template Header field and what, if any, needs to be added to the site’s CSS file.

Much appreciated.

https://wordpress.org/plugins/wp-catalogue/