PS: the default $styles array is this

$styles = array();
$styles['ingrinfo'] 	= 'white-space: initial; margin: 3px; line-height: 130%;' ;
$styles['ingr_item'] 	= 'font-style: italic;' ;
$styles['ingrinfo'] 	= 'white-space: initial; margin: 3px 3px 3px 20px; line-height: 130%; font-size:85%;' ;
$styles['ingrgrp'] 	= 'white-space: initial; margin: 3px;line-height: 130%;' ;
$styles['ingrgrp_0'] 	= 'white-space: initial; margin: 3px;line-height: 130%; display:inline' ;
$styles['ingrgrp_1'] 	= 'white-space: initial; margin: 3px;line-height: 130%; margin-left: 15px' ;
$styles['ingrgrp_lbl'] 	= 'font-weight: bold; padding-right: 3px;' ;
$styles['comments'] 	= 'padding: 3px 0; font-size: 75%;' ;

1)
you MUST learn how wordpress action and filter hooks work if you want to do ANY kind of wordpress alteration that are beyond just simple point and click
here’s a quick overview

https://docs.wp-pizza.com/developers/?section=filters-actions-functions

2) it depends on where you put your code (i strongly suggest you use child themes as they do not get updated when you update your theme)

the reason why it’s not in the template section is because it’s coming from the add ingredients plugin and the options that plugin provides to add things to orders would make it completely unmanageble if added to the inputs of th etemplates

your filter will the look *something like* this (depending what exactly you want to achieve, but as a starting point)

add_filter('wppizza_ingredients_filter_email_styles', 'myprefix_filter');
function myprefix_filter($styles){
/* 
the 120% here overrides any previous (default of 85%) 
font-size declaration for the element   
*/
$styles[ingrinfo] .= 'font-size:120%;';
return $styles;
}