fix(editor): Allow overriding theme from query params (#7591)

Allow overriding theme through query params.. to be able to override it
from preview iframe in webcomponent

Github issue / Community forum post (link here to close automatically):

cypress/e2e/31-demo.cy.ts

@@ -0,0 +1,23 @@
+import workflow from '../fixtures/Manual_wait_set.json';
+import { importWorkflow, vistDemoPage } from '../pages/demo';
+import { WorkflowPage } from '../pages/workflow';
+
+const workflowPage = new WorkflowPage();
+
+describe('Demo', () => {
+	it('can import template', () => {
+		vistDemoPage();
+		importWorkflow(workflow);
+		workflowPage.getters.canvasNodes().should('have.length', 3);
+	});
+
+	it('can override theme to dark', () => {
+		vistDemoPage('dark');
+		cy.get('body').should('have.attr', 'data-theme', 'dark');
+	});
+
+	it('can override theme to light', () => {
+		vistDemoPage('light');
+		cy.get('body').should('have.attr', 'data-theme', 'light');
+	});
+});