Add dark mode

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

assets/sass/header.scss

@@ -2,7 +2,8 @@
   padding: 2rem;
 
   .header-container {
-    max-width: $maxwidth;
+    max-width: var(--width-max);
+
     width: 100%;
     margin: 0 auto;
 
@@ -47,7 +48,7 @@
   }
 }
 
-@media (max-width: $mobilewidth) {
+@media (max-width: var(--width-mobile)) {
   $nav-toggle-label-height: 0.75rem;
   $nav-toggle-label-width: 1.5rem;
   $nav-toggle-span-height: 0.125rem;
@@ -69,7 +70,7 @@
         justify-content: center;
 
         opacity: 0;
-        background: $color-bg;
+        background: var(--color-bg);
         transition: opacity 0.3s ease;
 
         pointer-events: none;