If you have a locally downloaded theme it can be installed by dragging it into the Add-ons Themes window. Seems simple enough but if you don’t have a graphical file manager on the machine then you may not have an obvious way to drag the file.  All hope is not lost though.

Firefox’s own open-file dialog can act as a file manager allowing you to drag the file.

Steps:

1. Open the Add-ons window and select Themes at the top
2. Open Firefox’s open-file dialog by selecting “File->Open File…” from the menu
3. Browse to your locally downloaded theme then drag it from the Open File dialog to the Add-ons/Themes window
4. It should install at this point
5. Cancel/close the Open File dialog