Ok, so I've choose "white" theme while creating a presentation. Can I somehow add slides from black theme? Or can I change theme of my presentation?
It doesn't look like you can do it in Keynote 2013 as Apple has removed this feature with this version.
An idea that you could do is to make a new Keynote file with the secondary theme that you want, copy the slide, and then paste the slide into your presentation.
Yep, you can change the theme of your Keynote presentation in Keynote '13:
Go to the Presentation Setup Inspector (View → Inspector → Presentation Setup).
Click the Presentation tab.
Click Change Theme.
Select your new theme and click Choose.