So, I too changed My Settings to "Mobile Style" just to see what would happen. It sucks bad, at least for a PC. I don't recommend it.
But getting out of "Mobile Style" is not obvious or intuitive. The easiest way I found to do it is to go down to the lower left window on the page with the style options scroll box, and select anything BUT "Default Style" from the menu. (That only gives you the default view of the forum, but you go right back into Mobile Style again. No idea why. My mind. It is blown. Again.)
Anyway, so I did that -- went down to the window at the bottom left and selected "Large Font", and I got large fonts.
From there I could then select Default Style and bring back the full site layout again, and it stuck. Whew.
Note that when in "Mobile Style", the "Full Site" option at the bottom of the page is temporary, only for the page you are on.
You will still have to go back into "My Settings" to change back to default settings, but at least now you'll be able to do that.