Replace more hardcoded css

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

@@ -177,10 +177,10 @@
 .epoch-blocks {
   display: block;
   width: 100%;
-  background: #2d3348;
+  background: var(--secondary);
 
   .rect {
-    fill: #2d3348;
+    fill: var(--secondary);
 
     &.behind {
       fill: #D81B60;
@@ -189,7 +189,7 @@
       fill: url(#diff-gradient);
     }
     &.ahead {
-      fill: #1a9436;
+      fill: var(--success);
     }
 
     &.hover {
@@ -223,12 +223,12 @@
     height: 100%;
   }
   .background {
-    background: linear-gradient(to right, #105fb0, #9339f4);
+    background: linear-gradient(to right, var(--primary), #9339f4);
     left: 0;
     right: 0;
   }
   .remaining {
-    background: #2d3348;
+    background: var(--secondary);
     right: 0;
   }
   .label {