fix(theme): property() mixin not working with theme key strings

Fixes #6158

PiperOrigin-RevId: 318833674

packages/mdc-theme/_mixins.scss

@@ -91,7 +91,7 @@
   } @else if map.has-key(variables.$property-values, $value) {
     // $value is a theme property String
     $fallback: map.get(variables.$property-values, $value);
-    $custom-prop: custom-properties.create(--mdc-theme-#{$style}, $fallback);
+    $custom-prop: custom-properties.create(--mdc-theme-#{$value}, $fallback);
     @include custom-properties.apply(
       $property,
       $custom-prop,

packages/mdc-theme/test/theme.scss.test.ts

@@ -0,0 +1,38 @@
+/**
+ * @license
+ * Copyright 2020 Google Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+
+describe('theme.test.scss', () => {
+  it('should transform theme keys to custom property for theme.property()',
+     () => {
+       const filePath = path.join(__dirname, 'theme.test.css');
+       const css = fs.readFileSync(filePath, 'utf8').trim();
+       expect(css).toEqual(`.test {
+  color: #6200ee;
+  /* @alternate */
+  color: var(--mdc-theme-primary, #6200ee);
+}`);
+     });
+});

packages/mdc-theme/test/theme.test.scss

@@ -0,0 +1,5 @@
+@use "../mixins" as theme;
+
+.test {
+  @include theme.property(color, primary);
+}