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/Gantt/index.ts

@@ -22,8 +22,11 @@ import controlPanel from './controlPanel';
 import buildQuery from './buildQuery';
 import { EchartsChartPlugin } from '../types';
 import thumbnail from './images/thumbnail.png';
+import thumbnailDark from './images/thumbnail-dark.png';
 import example1 from './images/example1.png';
+import example1Dark from './images/example1-dark.png';
 import example2 from './images/example2.png';
+import example2Dark from './images/example2-dark.png';
 
 export default class EchartsGanttChartPlugin extends EchartsChartPlugin {
   constructor() {
@@ -46,7 +49,11 @@ export default class EchartsGanttChartPlugin extends EchartsChartPlugin {
         ),
         tags: [t('ECharts'), t('Featured'), t('Timeline'), t('Time')],
         thumbnail,
-        exampleGallery: [{ url: example1 }, { url: example2 }],
+        thumbnailDark,
+        exampleGallery: [
+          { url: example1, urlDark: example1Dark },
+          { url: example2, urlDark: example2Dark },
+        ],
       },
       transformProps,
     });