Sophia here from Shopify.
In the Theme Store, we can filter the themes by number of products, layout style, page features, and more. The first free theme that comes to mind for a charity website is Jumpstart because it has a built-in goal-tracking tool that you could use to showcase how much you've raised towards a funding goal, but the theme you choose really depends on the features you're looking for.
I'd love to learn more about the charity you're building a Shopify store for. Are you going to be accepting donations through the website? Will you be selling merchandise or other products to support the charity? If there are any particular features you're looking for in the website, let me know and I'd be happy to help you narrow down a few themes that would be a good fit.