change hardcoded colors to variable

assets/scss/pages/post.scss

@@ -195,7 +195,7 @@ $tocBreakpoint: 1024px;
 footer {
   text-align: center;
   padding: 0 1.5rem;
-  background: #ffffff;
+  background: $white;
 
   p {
     margin-top: 1rem; // reduce margin top due to social icons' padding

assets/scss/partials/_burger.scss

@@ -5,7 +5,7 @@
   padding: 0 1.5rem;
   position: fixed;
   width: 100%;
-  background: #fff;
+  background: $white;
   z-index: 2;
 
   @media screen and (min-width: $medium) {

assets/scss/partials/_nav.scss

@@ -6,7 +6,7 @@
   display: flex;
   justify-content: center;
   align-items: center;
-  background: #fff;
+  background: $white;
   visibility: hidden;
   z-index: 1;