Add website theme section to basics

docs/websites/website-basics.qmd

@@ -154,6 +154,19 @@ When creating links between pages in your site, you can provide the source file
 
 One benefit of using this style of link as opposed to targeting `.html` files directly is that if you at some point convert your site to a [book](/docs/books/book-basics.qmd) the file-based links will automatically resolve to section links for formats that produce a single file (e.g. PDF or MS Word).
 
+## Website Theme
+
+The look of your website is controlled via [HTML themes](/docs/output-formats/html-themes.qmd). These options are set under the `format` option in your website configuration file:
+
+```{.yaml filename="_quarto.yml"}
+format:
+  html:
+    theme: cosmo
+    css: styles.css
+```
+
+See [HTML Theming](/docs/output-formats/html-themes.qmd) for details on the available options.
+
 ## Learning More
 
 Once you've got a basic website up and running check out these articles for various ways to enhance your site: