Sidebar padding

how to make my modify sidebar content padding width custom option for 10px padding in theme child?

file: theme-options.php
row 921

before:

array(
	'value' => '30',
	'label' => '280px primary, 200px secondary (30px padding)'
	),
array(
	'value' => '20',
	'label' => '300px primary, 220px secondary (20px padding)'
)

after:

array(
	'value' => '30',
	'label' => '280px primary, 200px secondary (30px padding)'
),
array(
	'value' => '20',
	'label' => '300px primary, 220px secondary (20px padding)'
),
array(
	'value' => '10',
	'label' => 'custom (10px padding)'
)