chore(e2e): switch to Sass theme instead of prebuilt (#15690)

This is in preparation for upcoming changes to the e2e app

Author: mmalerba

src/e2e-app/BUILD.bazel

@@ -1,5 +1,6 @@
 package(default_visibility=["//visibility:public"])
 
+load("@io_bazel_rules_sass//:defs.bzl", "sass_binary")
 load("@npm_bazel_typescript//:defs.bzl", "ts_devserver")
 load("//tools:defaults.bzl", "ng_module")
 load("//:packages.bzl", "ANGULAR_LIBRARY_UMDS")
@@ -26,6 +27,14 @@ ng_module(
   ],
 )
 
+sass_binary(
+  name = "theme",
+  src = "theme.scss",
+  deps = [
+    "//src/lib/core:all_themes"
+  ]
+)
+
 ts_devserver(
     name = "devserver",
     port = 4200,
@@ -47,6 +56,7 @@ ts_devserver(
       "@npm//@webcomponents/custom-elements",
       "//src/lib/prebuilt-themes:indigo-pink",
       ":index.html",
+      ":theme",
     ],
     # Scripts which will be included in the serving_path bundle after "require.js" has been
     # loaded.

src/e2e-app/index.html

@@ -9,7 +9,7 @@
   <link rel="icon" type="image/x-icon" href="favicon.ico">
   <link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
   <link href="https://fonts.googleapis.com/css?family=Roboto:300,400,500" rel="stylesheet">
-  <link href="angular_material/src/lib/prebuilt-themes/indigo-pink.css" rel="stylesheet">
+  <link href="theme.css" rel="stylesheet">
 
   <!-- FontAwesome for mat-icon demo. -->
   <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css">

src/e2e-app/theme.scss

@@ -0,0 +1,16 @@
+@import '../lib/core/theming/all-theme';
+
+// Plus imports for other components in your app.
+
+// Include the common styles for Angular Material. We include this here so that you only
+// have to load a single css file for Angular Material in your app.
+// **Be sure that you only ever include this mixin once!**
+@include mat-core();
+
+// Define the default theme (same as the example above).
+$candy-app-primary: mat-palette($mat-indigo);
+$candy-app-accent:  mat-palette($mat-pink, A200, A100, A400);
+$candy-app-theme:   mat-light-theme($candy-app-primary, $candy-app-accent);
+
+// Include the default theme styles.
+@include angular-material-theme($candy-app-theme);