fix not rendered style when style is optional

packages/mermaid/src/diagram-api/diagramAPI.ts

@@ -7,7 +7,6 @@ import { addStylesForDiagram } from '../styles.js';
 import { DiagramDefinition, DiagramDetector } from './types.js';
 import * as _commonDb from '../commonDb.js';
 import { parseDirective as _parseDirective } from '../directiveUtils.js';
-import isEmpty from 'lodash-es/isEmpty.js';
 
 /*
   Packaging and exposing resources for external diagrams so that they can import
@@ -51,9 +50,7 @@ export const registerDiagram = (
   if (detector) {
     addDetector(id, detector);
   }
-  if (!isEmpty(diagram.styles)) {
-    addStylesForDiagram(id, diagram.styles);
-  }
+  addStylesForDiagram(id, diagram.styles);
 
   if (diagram.injectUtils) {
     diagram.injectUtils(

packages/mermaid/src/styles.ts

@@ -1,7 +1,7 @@
 import type { FlowChartStyleOptions } from './diagrams/flowchart/styles.js';
 import { log } from './logger.js';
 
-const themes: Record<string, any> = {};
+const themes: Record<string, (options?: any) => string> = {};
 
 const getStyles = (
   type: string,
@@ -73,8 +73,13 @@ const getStyles = (
 `;
 };
 
-export const addStylesForDiagram = (type: string, diagramTheme: unknown): void => {
-  themes[type] = diagramTheme;
+export const addStylesForDiagram = (
+  type: string,
+  diagramTheme?: (options?: any) => string
+): void => {
+  if (diagramTheme !== undefined) {
+    themes[type] = (options) => diagramTheme(options);
+  }
 };
 
 export default getStyles;