fix(demo): only include the theme styles for the components that we use

- this cuts out about 40 KB of minified CSS

Author: Splaktar

src/styles.scss

@@ -9,7 +9,12 @@
 // Include theme styles for core and each component used in your app.
 // Alternatively, you can import and @include the theme mixins for each component
 // that you are using.
-@include angular-material-theme($demo-theme);
+@include mat-core-theme($demo-theme);
+@include mat-icon-theme($demo-theme);
+@include mat-menu-theme($demo-theme);
+@include mat-button-theme($demo-theme);
+@include mat-toolbar-theme($demo-theme);
+@include mat-tooltip-theme($demo-theme);
 
 // Pass the theme into the @devintent/dev components.
 @include dev-theme($demo-theme);