Custom CSS Editor
The Custom CSS Editor ( custom CSS Field/box ) allows you to customize the appearance of your theme without the need to create a child theme or worry about theme updates overwriting your customizations.
Head to Customize Section: Dashboard (wp-admin) >> Appearance >> customize >>header settings >> custom CSS (Custom CSS Editor)
Screenshot view.
Note: Above custom CSS editor path varies with different themes. Here you got the idea about custom CSS box.
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