[llanita]

Go to Settings/Language & Input. Click on Google Keyboard and you'll get Google Keyboard preferences.
Click on Appearance and Layout. Click on Theme and pick whichever one of the four options you prefer. Holo Blue is the dark keyboard.
You can also choose other options from the list in Google Keyboard Preferences
Hope this helps.

[daihard2008]

Go to Settings/Language & Input. Click on Google Keyboard and you'll get Google Keyboard preferences.
Click on Appearance and Layout. Click on Theme and pick whichever one of the four options you prefer. Holo Blue is the dark keyboard.
You can also choose other options from the list in Google Keyboard Preferences
Hope this helps.

Different keyboard themes have been available for quite some time. They just added two Material Design themes with the introduction of Lollipop. I use the Material Dark theme both on my English and Japanese keyboards.

[llanita]

I made this post for those who didn't know about the options, because some were complaining that they didn't like the white keyboard and in case they didn't know how to switch it.

[sibber]

On the galaxy 5s with Lollipop, no google keyboard. Just samsung. and no themes. Any idea on how to switch back to dark keyboard?