Misc cleanup, more definitions of color with scss variables and added a lot of color transitions for when switching theme color.

src/App.vue

@@ -97,10 +97,14 @@ body{
   line-height: 1.6;
   background: $background-color;
   color: $text-color;
+  transition: background-color .5s ease, color .5s ease;
   &.hidden{
     overflow: hidden;
   }
 }
+h1,h2,h3 {
+  transition: color .5s ease;
+}
 a:any-link {
   color: inherit;
 }