Save Pages Shortcuts For All Documents
Although it is not possible to change for ALL documents, you can create a custom template that preserves the shortcuts for future documents based on that template.


Set your shortcuts in a new document, preferably one you already have formatted the way you like it.
Click File > Save as Template...
Save in Template chooser and name the Template.
Use this new template (Found under My Templates whenever you wish to use those particular shortcuts.


Of course, this will only preserve the shortcuts for that particular template and all documents based on it, not universally with other documents.