diff --git a/packages/block-editor/src/components/iframe/index.js b/packages/block-editor/src/components/iframe/index.js
index e59e564da3e6a..c4454d17ff003 100644
--- a/packages/block-editor/src/components/iframe/index.js
+++ b/packages/block-editor/src/components/iframe/index.js
@@ -141,6 +141,13 @@ function Iframe( {
 		function onLoad() {
 			const { contentDocument, ownerDocument } = node;
 			const { documentElement } = contentDocument;
+			// Get any CSS classes the iframe document body may initially have
+			// to re-apply them later together with the ones of the main document
+			// body. This is necessary for some CSS classes for example the
+			// `is-dark-theme` class added by useDarkThemeBodyClassName.
+			const initialIframeBodyClasses = Array.from(
+				contentDocument.body.classList
+			);
 			iFrameDocument = contentDocument;
 
 			documentElement.classList.add( 'block-editor-iframe__html' );
@@ -151,12 +158,15 @@ function Iframe( {
 			// be added in the editor too, which we'll somehow have to get from
 			// the server in the future (which will run the PHP filters).
 			setBodyClasses(
-				Array.from( ownerDocument.body.classList ).filter(
-					( name ) =>
-						name.startsWith( 'admin-color-' ) ||
-						name.startsWith( 'post-type-' ) ||
-						name === 'wp-embed-responsive'
-				)
+				Array.from( ownerDocument.body.classList )
+					.concat( initialIframeBodyClasses )
+					.filter(
+						( name ) =>
+							name.startsWith( 'admin-color-' ) ||
+							name.startsWith( 'post-type-' ) ||
+							name === 'wp-embed-responsive' ||
+							name === 'is-dark-theme'
+					)
 			);
 
 			contentDocument.dir = ownerDocument.dir;