Added a link to the Theme above.
You need to go to Admin-> Customise-> Themes
If adding just the CSS for the button create a Theme Component and edit the CSS for Desktop.
Add the component to an existing Theme.
Beginners guide to Themes here.
If all this is just too complex or long winded, just add the Theme I linked above following the Beginners guide 
(PS just fixed the CSS above which was missing some dependencies)