1. By default, you'll find any custom style sets in the C:\Users\Your_User_Name_Here\AppData\Roaming\Micro soft\QuickStyles folder. They are saved as *.dotx files, and the names are identical to the style set names. (The built-in ones are in the program files folder.)

2. If you are adding a style set to the Normal template, using Home | Style Sets | Set as Default should do the trick. Just remember to save the template if you see a prompt.

Whenever you want to add a style set to a custom template, open it instead. And, yes, you should open the template as a document, so that you see its filename on the title bar (not "Document 1" or something similar).

Important note: Numbered and bulleted styles may not work properly when you add them to a style set. Number/bullet formatting may not be predictable.

Since the style set is simply the set of styles displayed on the Home tab for a given document or template, what you see will be different depending on the open document. In particular, if you modify the style set for a template, existing documents based on that template won't be affected.

You can always choose to set up a template instead, with the desired styles, and ignore style sets.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional