feat: Add comprehensive dark mode support for chart thumbnails and examples (#35111)

Co-authored-by: Claude <noreply@anthropic.com>

superset-frontend/plugins/plugin-chart-echarts/src/Tree/index.ts

@@ -20,7 +20,9 @@ import { t } from '@superset-ui/core';
 import controlPanel from './controlPanel';
 import transformProps from './transformProps';
 import thumbnail from './images/thumbnail.png';
+import thumbnailDark from './images/thumbnail-dark.png';
 import example from './images/tree.png';
+import exampleDark from './images/tree-dark.png';
 import buildQuery from './buildQuery';
 import { EchartsChartPlugin } from '../types';
 
@@ -36,7 +38,7 @@ export default class EchartsTreeChartPlugin extends EchartsChartPlugin {
         description: t(
           'Visualize multiple levels of hierarchy using a familiar tree-like structure.',
         ),
-        exampleGallery: [{ url: example }],
+        exampleGallery: [{ url: example, urlDark: exampleDark }],
         name: t('Tree Chart'),
         tags: [
           t('Categorical'),
@@ -47,6 +49,7 @@ export default class EchartsTreeChartPlugin extends EchartsChartPlugin {
           t('Featured'),
         ],
         thumbnail,
+        thumbnailDark,
       },
       transformProps,
     });