convert to CSS variables

frontend/src/app/lightning/nodes-per-isp-chart/nodes-per-isp-chart.component.html

@@ -33,7 +33,7 @@
         <fa-icon [icon]="['fas', 'download']" [fixedWidth]="true"></fa-icon>
       </button>
     </div>
-    <small class="d-block" style="color: #ffffff66; min-height: 25px" >
+    <small class="d-block" style="color: var(--transparent-fg); min-height: 25px" >
       <span i18n="lightning.tor-nodes-excluded">(Tor nodes excluded)</span>
     </small>
   </div>

frontend/src/app/lightning/nodes-per-isp-chart/nodes-per-isp-chart.component.scss

@@ -97,7 +97,7 @@
     .card-text {
       font-size: 18px;
       span {
-        color: #ffffff66;
+        color: var(--transparent-fg);
         font-size: 12px;
       }
     }

frontend/src/app/lightning/nodes-per-isp-chart/nodes-per-isp-chart.component.ts

@@ -275,7 +275,7 @@ export class NodesPerISPChartComponent implements OnInit {
 
   onSaveChart(): void {
     const now = new Date();
-    this.chartOptions.backgroundColor = '#11131f';
+    this.chartOptions.backgroundColor = 'var(--active-bg)';
     this.chartInstance.setOption(this.chartOptions);
     download(this.chartInstance.getDataURL({
       pixelRatio: 2,