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/Sankey/index.ts

@@ -22,8 +22,11 @@ import buildQuery from './buildQuery';
 import controlPanel from './controlPanel';
 import transformProps from './transformProps';
 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';
 import { SankeyChartProps, SankeyFormData } from './types';
 
 // TODO: Implement cross filtering
@@ -55,10 +58,14 @@ export default class EchartsSankeyChartPlugin extends ChartPlugin<
           height corresponds to the visualized metric, providing a clear representation of
           value distribution and transformation.`,
         ),
-        exampleGallery: [{ url: example1 }, { url: example2 }],
+        exampleGallery: [
+          { url: example1, urlDark: example1Dark },
+          { url: example2, urlDark: example2Dark },
+        ],
         name: t('Sankey Chart'),
         tags: [t('Directional'), t('ECharts'), t('Distribution'), t('Flow')],
         thumbnail,
+        thumbnailDark,
       }),
       transformProps,
     });