diff --git a/lib/types/library/LibraryBuilder.js b/lib/types/library/LibraryBuilder.js
index c18afcc5e..8e31cabcc 100644
--- a/lib/types/library/LibraryBuilder.js
+++ b/lib/types/library/LibraryBuilder.js
@@ -154,7 +154,8 @@ class LibraryBuilder extends AbstractBuilder {
 		this.addTask("buildThemes", async () => {
 			// Only compile themes directly below the lib namespace to be in sync with the theme support at runtime
 			// which only loads themes from that folder.
-			const inputPattern = `/resources/${project.metadata.namespace}/themes/*/library.source.less`;
+			// TODO 3.0: Remove fallback in case of missing namespace
+			const inputPattern = `/resources/${project.metadata.namespace || "**"}/themes/*/library.source.less`;
 
 			return getTask("buildThemes").task({
 				workspace: resourceCollections.workspace,