Pages comes with a default text box style, but you can create your own default text box style for a document. Your default text box style applies only to the document where you added it.
Add a text box to your document and change it however you want.
You can change the font and font size, for example, add a border to the box, and so on.
Select the text box.
Choose Format > Advanced > Set as Default Text Box Appearance (from the Format menu at the top of your screen).
You can change the default whenever you want, and it won’t affect any text boxes already in the document.
If you want to apply the same design changes to other text boxes that already exist in the document, you can save the default text box style as an object style, then apply the object style to other text boxes.