Can I try a theme with my own products before activating it?

Yes. Go to Settings → Appearance → Themes, hover over a theme card and click Quick Preview. The Live Simulator opens:

• Theme Demo shows the theme with its own demo products and pages.
• My Store shows the same theme with your own products, categories, menus and logo, on every page.
• Desktop, Tablet and Mobile switch the screen size.

Nothing is applied to your store while you preview, so customers keep seeing your current theme. When you're happy, close the preview and click Install, then Activate.