Adopt theming system of Angular Material

The SASS has been simplified so that different themes can be applied by
only using the $mdThemingProvider serivce.

The resulting design is more "Materialized" and less prone to the
future changes of Angular Material.

UI/WebServerResources/scss/components/content/content-theme.scss

@@ -0,0 +1,3 @@
+.sg-subcontent {
+  border-left: $baseline-grid solid sg-color($sogoGreen, 100);
+}