uiux(login): introduce dark-theme for login screen

This introduces a new dark-theme file for the login screen
and ensures that it keeps the contrast ratio in tact when
switching themes.

Notice that it simply overrides the component styles which are
expected to be light-theme by default. That's because we don't
maintain equivalent SCSS files for the light-theme at the moment.

Fixes #114

embark-ui/src/dark-theme/_login.scss

@@ -0,0 +1,12 @@
+.login-container {
+  color: $gray-900;
+  box-shadow: 0px 5px 20px $gray-800;
+
+  .login-layout-container-section {
+    background: $gray-900;
+
+    &:first-child {
+      background: $white;
+    }
+  }
+}

embark-ui/src/dark-theme/coreui.scss

@@ -44,6 +44,7 @@
   @import "switches";
   @import "tables";
   @import "widgets";
+  @import "login";
 
   // Layout Options
   @import "layout";