For layout issues you shouldn’t have to edit any PHP files, use cases to fix any issues.
You can add the styles to your theme or include a new stylesheet to fix the issues. Also if you want to complete restyle all the image store pages you can deactivate the default stylesheet under settings > general > Use CSS.
If you still want to modify the files we suggest using a secondary plugin so that you can update the plugin for future leases.
Use the filter “ims_order_form_path” to use the modified version