Pilcrow Featured Image Size

Hi,

I am using the Pilcrow theme and am new to wordpress but have some experience with php.

The 770 width is too narrow so I went in to the editor and changed styles.css to 800. However, that caused my featured images not to go to across the entire 800 pixels (they were still at 770 but the black lines bordering it extended farther out).

I then went in and tried changing:
‘$args[‘width’] = apply_filters( ‘pilcrow_header_image_width’, 770);’ in custom-header.php but then all the my featured images change to just the default header image (the books).

How do I set wider featured images? Thank you so much for your help!

Kind regards,
Greg