As far as I know there's no way to redefine it back to the hyperlink colourtheme colour using the Style Modify dialog. However here are two methods you can try.
VBA Method
Alt + F12F11 to open up VBA Window. Alt + G to open Immediate Window. Then run the following command in the Immediate Window:
ActiveDocument.Styles("Hyperlink").Font.TextColor.ObjectThemeColor = wdThemeColorHyperlink
Style painter method
You can use style painter to copy the formatting back and use that to update the Hyperlink style.
- Create a new document type some text, ensure it's formatted as Normal and then format it as hyperlink
- Select that text and press Ctrl + Shift + C (this will copy the formatting)
- In your target document type some new text to work with, format it as Normal and apply Hyperlink to it
- Select the new text and press Ctrl + Shift + P (this will paste the formatting)
- If it's not already visible open the Styles Window (Ctrl + Alt + Shift + S)
- Locate the Hyperlink style in the style window
- Make sure your hyperlink formatted text is still selected
- Hover over it and then click the drop down menu and select "Update Hyperlink to Match Selection"
- Delete the working text you added in step 3
You should now have the hyperlink text back to it's defaults. Check the other Hyperlink text in your document. If you had any other modifications to the Hyperlink style you can now re-apply those.