convert to CSS variables

frontend/src/app/lightning/node-statistics-chart/node-statistics-chart.component.ts

@@ -181,7 +181,7 @@ export class NodeStatisticsChartComponent implements OnInit {
           splitLine: {
             lineStyle: {
               type: 'dotted',
-              color: '#ffffff66',
+              color: 'var(--transparent-fg)',
               opacity: 0.25,
             }
           },
@@ -217,7 +217,7 @@ export class NodeStatisticsChartComponent implements OnInit {
             symbol: 'none',
             lineStyle: {
               type: 'solid',
-              color: '#ffffff66',
+              color: 'var(--transparent-fg)',
               opacity: 1,
               width: 1,
             },