What is a WordPress theme?

The theme is what your site looks like: the layout, the fonts, the colours, where the menu sits, how a blog post is arranged on the page.

Your content — pages, posts, images, products — is stored separately, in the database. That separation is the useful part: changing theme changes the appearance and leaves the content where it is.

One theme is active at a time. You can have several installed; only the active one is doing anything.

What changing theme does and does not do

Does not touch: your pages, posts, images, products, orders, users, or your domain.

Does change: the layout of everything, and anything the old theme provided — its widgets, its menu positions, its custom page templates, and any settings that lived in its own options panel.

That is why a theme swap on a site that has been running for years is disruptive even though nothing is deleted. Try it on a staging copy first.

Block themes and classic themes

Newer themes are block themes: you edit the whole site, headers and footers included, with the same block editor you use for a page.

Classic themes are edited through Appearance → Customise and their own options screens, and page layout is usually left to a page builder plugin.

Neither is better. Whichever you pick, stay with its way of working rather than fighting it with a plugin that wants to do the same job.

Child themes

Editing a theme's files directly means your changes are wiped the next time it updates. A child theme is a small theme that inherits everything from the parent and holds only your changes, so updates are safe.

If you are pasting CSS, you do not need one — use Appearance → Customise → Additional CSS, which survives updates on its own.

Next: install and activate a theme.