convert to CSS variables

frontend/src/app/components/pool/pool-preview.component.scss

@@ -17,7 +17,7 @@
     justify-content: space-between;
     width: 100%;
     margin-left: 15px;
-    background: #181b2d;
+    background: var(--stat-box-bg);
     padding: 0.75rem;
     width: 0;
     flex-grow: 1;
@@ -43,7 +43,7 @@
 .chart {
   width: 100%;
   height: 315px;
-  background: #181b2d;
+  background: var(--stat-box-bg);
 }
 
 .row {
@@ -65,7 +65,7 @@
     position: absolute;
     right: 0;
     top: 0;
-    background: #24273e;
+    background: var(--box-bg);
 
     &.noimg {
       opacity: 0;

frontend/src/app/components/pool/pool.component.scss

@@ -88,7 +88,7 @@ div.scrollable {
 }
 
 .progress {
-  background-color: #2d3348;
+  background-color: var(--secondary);
 }
 
 .coinbase {