You can create as many themes as you like, and one per Storefront, or one for multiple Storefronts.

To do this either go to

Admin>Storefront Settings >Appearance>Theme>Edit Theme

or

Admin>System Settings>Themes>Edit Theme

Then scroll down on the Theme Preview and CLICK ANYWHERE

In the pic above, I've clicked on the product card, and the Product theme options has opened on the left.

Changing the colour of an object, affects the preview, in this case, the buttons on the cards.

You can also select object groups by using the drop down on the left.

Make sure you scroll through the whole preview to see how its affected by your changes.

Remember to Save your changes, and also CHECK the appearance of the front end by logging in as a Customer.