Document how to use a custom Shiki theme (#1749)

Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>
Co-authored-by: Kevin Zuniga Cuellar <46791833+kevinzunigacuellar@users.noreply.github.com>

src/pages/en/guides/markdown-content.md

@@ -581,7 +581,22 @@ export default {
 };
 ```
 
-We also suggest [diving into their theme documentation](https://github.com/shikijs/shiki/blob/main/docs/themes.md#loading-theme) to explore loading custom theme, light vs dark mode toggles, or styling via CSS variables.
+#### Adding your own theme
+
+Instead of using one of Shiki’s predefined themes, you can import a custom theme from a local file.
+
+```js title="astro.config.mjs"
+import { defineConfig } from 'astro/config';
+import customTheme from './my-shiki-theme.json';
+
+export default defineConfig({
+  markdown: {
+    shikiConfig: { theme: customTheme },
+  },
+});
+```
+
+We also suggest [diving into their theme documentation](https://github.com/shikijs/shiki/blob/main/docs/themes.md#loading-theme) to explore more about themes, light vs dark mode toggles, or styling via CSS variables.
 
 #### Prism configuration