This setting will set the default tuning to songs to a specific instrument. It will not affect the instrument used for songbooks, since these are set for each book.
Chordie have two modes to display songs. The old mode is "table"-mode. The new mode is "css"-mode. "css"-mode works a lot better on mobile devices, and it is therefore the default rendering mode. You can however switch back to the old mode as well. Please note that the songs in your songbook may not be affected by this setting.