fix: add custom theme using string ref in angular.json (#11189)

Fixes #11188

Author: jerryorta-dev

src/lib/schematics/utils/ast.ts

@@ -67,7 +67,7 @@ export function getStylesPath(host: Tree, project: Project): string {
   const buildTarget = project.architect['build'];
 
   if (buildTarget.options && buildTarget.options.styles && buildTarget.options.styles.length) {
-    const styles = buildTarget.options.styles.map(s => s.input);
+    const styles = buildTarget.options.styles.map(s => typeof s === 'string' ? s : s.input);
 
     // First, see if any of the assets is called "styles.(le|sc|c)ss", which is the default
     // "main" style sheet.