Where do these steps being with the browser open on Facebook. But the themes will not appear.
Just click the arrow icon next to the Greasemonkey plugin (usually the icon is a monkey). Will open the admin panel. Theme endo you’re shown is marked on the side. Just click on it and ready. Now just click F5 to refresh the page and the theme will no longer appear.
To reactivate just do the same procedure and select a theme you want.
In Google Chrome:
To control the themes in Chrome Plugin Tampermonkey click the icon (usually located next to the address bar). Will open the admin panel. The theme being displayed will appear marked with a green ball beside. Click on it and turns red. Thus the theme will be deactived. If you want to reactivate just click on the green theme and score.
Hope this helps.