It seems that the default stylesheets used by Mathematica's Front End cannot be edited like other user-created stylesheets. The following, for example, is the stylesheet for the Help documents of Mathematica:
hlpst = $InstallationDirectory <>"/SystemFiles/FrontEnd/StyleSheets/Wolfram/Reference.nb"
After being opened, it looks like a usual stylesheet window, except that its contents (such as font colours) cannot be edited at all.
So my question is: How can the stylesheet Reference.nb, in the folder shown above, be edited in a stylesheet window, just as a user-created stylesheet can?