How can I use SlidesCarnival themes in Google Slides?

On the template detail page, scroll until you see the template preview.
Click on the button under the presentation preview that says Use as Google Slides Theme.
A new window will open with a larger preview. If you like what you see, click on the Use template button in the top right corner.
A copy of the document will be added to your own Google Drive account and you will be able to edit, add or delete slides.