How can I add some (very) basic styles?
For starters, you might like to add a background color to your summary element, and then a border of the same color around the entirety of everything else. This ties the contents together, visually.
In order to get a border around the entirety of your non-summary content, put it all in a div (or some other containing element you deem appropriate):
<details> <summary> <h3>Here is your summary content</h3> </summary> <div> <p>We've put all our other content in a containing div.</p> <p>This way, we can put a border around (or otherwise style) this container.</p> </div> </details>