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/Radar/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.jpg';
+import example1Dark from './images/example1-dark.jpg';
 import example2 from './images/example2.jpg';
+import example2Dark from './images/example2-dark.jpg';
 import { EchartsRadarChartProps, EchartsRadarFormData } from './types';
 import { EchartsChartPlugin } from '../types';
 
@@ -57,7 +60,10 @@ export default class EchartsRadarChartPlugin extends EchartsChartPlugin<
         description: t(
           'Visualize a parallel set of metrics across multiple groups. Each group is visualized using its own line of points and each metric is represented as an edge in the chart.',
         ),
-        exampleGallery: [{ url: example1 }, { url: example2 }],
+        exampleGallery: [
+          { url: example1, urlDark: example1Dark },
+          { url: example2, urlDark: example2Dark },
+        ],
         name: t('Radar Chart'),
         tags: [
           t('Business'),
@@ -69,6 +75,7 @@ export default class EchartsRadarChartPlugin extends EchartsChartPlugin<
           t('Featured'),
         ],
         thumbnail,
+        thumbnailDark,
       },
       transformProps,
     });