A theme is different from a template: the template gave you your sections and copy, the theme decides how the whole site looks — its fonts, its colour palette, its button and card shapes, its overall personality. Switching themes doesn't touch your content, so it's safe to try a few before settling.
Opening the theme picker
In the editor, open Site Settings › Colour Palette. The dialog opens on its Themes tab, which lists every site theme with a live preview. Click one to preview it against your actual site content, not a generic mockup — what you see is what you'll get.
Light and dark, and colour variations
Most themes support both light and dark mode, and several ship more than one colour variation of the same look. Once you've picked a theme, use the mode toggle and the colour swatches above the preview to fine-tune it before applying.
Applying it
Choosing a theme also lets you pick a starting page set — keep the pages you already have, or start from the theme's own recommended page list. If your theme's colours don't quite match your brand afterwards, the Colours tab in the same dialog lets you override individual site colours without leaving the theme itself.
A quicker way to try a theme on for size
Nothing you do in this dialog touches your live site until you submit — preview freely. If you're not sure, apply it, look at the real result, and switch again; there's no limit on how many times you can change themes.