if you wish to customise the css for this plugin you have several options

a) if your theme has a dedicated option to add css declarations directly on the pages, you can use that, overriding css declarations as required

b) create a wppizza-addingredients-custom.css file in your theme (or better still childtheme) directory and override/add css declarations as required

c) if you feel the need to edit ALL css declarations of the plugin, copy css/wppizza-addingredients.source.css OR css/wppizza-addingredients.min.css into your theme (or childtheme) directory
as wppizza-addingredients.css and edit as required

d) any other suitable place your theme offers to add your own css declarations

Note: Currently optional but will be required in wppizza v3.x:
Place customised css (and any other customised files or templates) into a “wppizza” subdirectory of your (child)theme (so the structure for customising the above css files would be /[child]theme-directory/wppizza/wppizza-addingredients.css)