Check out this old question:
How to stop V10 asking to continue with updated styling each time I open a new notebook
But what if you checked "do this for all notebooks", naively thinking that this would mean "do this for all notebooks for which this option to revert or update is presented/applicable"? Well, WRI support would lead you to believe that you might do something called a "clean start", but this is an obtuse solution, much like knocking down your entire house and rebuilding to fix a hole in your wall, except that this is actually like knocking down your neighbor's house too, if you lived in, say, Wolfram|One Desktop, and you also owned your neighbor's house, Mathematica. You will get a clean slate, but also a whole new house without all of the memories you created in there, and your neighbor will too! So, what can we do? Maybe a reinstallation of the walls of the house? Uninstall and reinstall them, if you will get my very rough analogous extension?
WELL ACTUALLY if you uninstall Wolfram Desktop, or Mathematica, and reinstall them, without deleting remaining folders present in the "clean start" method, you will still have a hole in your wall, EVEN AFTER INSTALLING THE NEW DRYWALL! What gives?! Why is this option, this change in default stylings (to which even copying over & overwriting with the stylesheets folder from unmodified Mathematica provides no solution or change to the default stylings) of notebooks, reinstall-hard?? Let it be known that this change in default styling options is present in both Wolfram|One Desktop and Mathematica, and so the proposed solution should be applicable to both, or sufficiently generalized, opposed to the help page for the "clean start" which merely indicates the solution applying to Mathematica.
So, what did I even ask here? Lets see:
- How can we undo the "do this for all notebooks" option, after checking it either by mistake or naivety?
- How can we make all old notebooks keep their old styling while keeping others updated, and to update the stylings for old ones, without affecting the updating/reverting of all notebooks?
Let's try to keep this to non-programmatic solutions, using options on the frontend, accessed through menu interactions!
GOOD LUCK! HAVE FUN!