test: Add test for edge marker color rendering

cypress/integration/rendering/flowchart.spec.js

@@ -917,4 +917,21 @@ graph TD
       }
     );
   });
+  it('#6369: edge color should affect arrow head', () => {
+    imgSnapshotTest(
+      `
+    flowchart LR
+        A --> B
+        A --> C
+        C --> D
+
+        linkStyle 0 stroke:#D50000
+        linkStyle 2 stroke:#D50000
+    `,
+      {
+        flowchart: { htmlLabels: true },
+        securityLevel: 'loose',
+      }
+    );
+  });
 });