There is more than one thing you can have a default on, so not sure which setting you're talking about but if it's the one I think it is, try this:
Click on My Profile
On the next page, that will look like this, click on the blue "customize my profile" button shown in this picture (lower right):
[ATTACH=CONFIG]482705[/ATTACH]
Then on the next page, click either the "revert" or the "default" button (towards the top to the left in the picture below)and see if that changes what you've changed.
Revert would take it back to whatever you had before if you'd changed a thing or two before the recent change, or default should take it back to how it was when the board got the new look a few years ago. If you didn't change anything but this one time, I'd say it wouldn't make any difference which you clicked, but just try it.
[ATTACH=CONFIG]482706[/ATTACH]
Let us know if this did the trick, or if it didn't, please post EXACTLY what has changed that you want back to default.
Good luck!