So your Google Slides presentation has loads of content in it, but you need that theme from the last one you did. So much effort and time went in to making that theme, but you don’t want to copy and paste all your content!
You could duplucate the old one, and then copy your content across. Or you could simply apply the theme from your last Slides doc!
Here are two ways to apply a theme.
1) a stock theme from Google boooooring.
2) Using a custom theme you have made previously – much more fun!