Add dark theme

Fixes #1151

assets/css/style.scss

@@ -177,6 +177,10 @@ h2, h3:not(.h5), h4, h5 {
   text-align: justify;
 }
 
+.no-text-wrap {
+  white-space: nowrap;
+}
+
 footer {
   img,
   i {
@@ -184,6 +188,7 @@ footer {
   }
 }
 
+
 /*
  * Navbar
  */
@@ -272,8 +277,15 @@ input#nav-toggle,
   display: none;
 }
 
+#nav-switch-theme {
+  /* We will make it visible with JavaScript
+   * as it does not work without it */
+  display: none;
+}
 
-/* Mobile hamburger menu */
+/*
+ * Mobile hamburger menu
+ */
 
 @media only screen and (max-width: 992px) {
   .menu,
@@ -336,6 +348,8 @@ input#nav-toggle,
   }
 }
 
-.no-text-wrap {
-  white-space: nowrap;
+.nav-theme-icon:before {
+  color: var(--warning);
+  font-size: 0.875em;
+  content: "\f186";
 }