Rewrite sass variables to support themes

src/sass/navbar.scss

@@ -4,12 +4,16 @@ nav {
     display: flex;
     align-items: center;
     position: fixed;
-    background-color: $bg_overlays;
+    background-color: var(--bg_overlays);
     box-shadow: 0 0 4px $shadow;
     padding: 0;
     width: 100%;
     height: 50px;
     z-index: 1000;
+
+    a, .icon-button button {
+        color: var(--fg_nav);
+    }
 }
 
 .inner-nav {
@@ -26,7 +30,7 @@ nav {
     font-weight: 600;
 
     &:hover {
-        color: $accent_light;
+        color: var(--accent_light);
         text-decoration: unset;
     }
 }
@@ -54,7 +58,7 @@ nav {
         padding-left: 4px;
 
         &:hover {
-            color: $accent_light;
+            color: var(--accent_light);
             text-decoration: unset;
         }
     }