convert to CSS variables

frontend/src/app/components/mempool-graph/mempool-graph.component.ts

@@ -432,7 +432,7 @@ export class MempoolGraphComponent implements OnInit, OnChanges {
         splitLine: {
           lineStyle: {
             type: 'dotted',
-            color: '#ffffff66',
+            color: 'var(--transparent-fg)',
             opacity: 0.25,
           }
         }
@@ -500,7 +500,7 @@ export class MempoolGraphComponent implements OnInit, OnChanges {
     const now = new Date();
     // @ts-ignore
     this.mempoolVsizeFeesOptions.grid.height = prevHeight + 20;
-    this.mempoolVsizeFeesOptions.backgroundColor = '#11131f';
+    this.mempoolVsizeFeesOptions.backgroundColor = 'var(--active-bg)';
     this.chartInstance.setOption(this.mempoolVsizeFeesOptions);
     download(this.chartInstance.getDataURL({
       pixelRatio: 2,