feat(theme): Add support for arbitrary CSS vars with fallback (#4470)

(cherry picked from commit 0bfb393)

packages/mdc-theme/README.md

@@ -110,6 +110,38 @@ Property Name | Description
 `on-secondary` | A text/iconography color that is usable on top of secondary color
 `on-surface` | A text/iconography color that is usable on top of surface color
 
+#### `mdc-theme-prop` with CSS Custom Properties
+
+> **Note** The Sass map `$style` argument is intended *only* for use with color mixins.
+
+The `mdc-theme-prop` mixin also accepts a Sass map for the `$style` argument. The map must contain the following fields:
+
+Fields | Description
+--- | ---
+`varname` | The name of a CSS custom property
+`fallback` | A fallback value for the CSS custom property
+
+For example, the following Sass...
+
+```
+.foo {
+  @include mdc-theme-prop(color, (
+    varname: --foo-color,
+    fallback: red,
+  ));
+}
+```
+
+...will produce the following CSS...
+
+```
+.foo {
+  color: red;
+  color: var(--foo-color, red);
+}
+```
+
+The above output CSS will apply the `fallback` field's value for all supported browsers (including IE11) while allowing for CSS custom property use as a progressive enhancement. Browsers like IE11 that do not support CSS custom properties will apply the `color: red;` and ignore the `color: var(--foo-color, red);`. This argument type is intended for clients who need custom color application outside of the existing theme properties.
 
 #### `mdc-theme-luminance($color)`
 

packages/mdc-theme/_functions.scss

@@ -64,3 +64,18 @@
 @function mdc-theme-contrast-tone($color) {
   @return if(mdc-theme-tone($color) == "dark", "light", "dark");
 }
+
+@function mdc-theme-is-var-with-fallback_($style) {
+  @return type-of($style) == "map" and map-has-key($style, "varname") and map-has-key($style, "fallback");
+}
+
+@function mdc-theme-get-var-fallback_($style) {
+  @return map-get($style, "fallback");
+}
+
+@function mdc-theme-var_($style) {
+  $var: map-get($style, "varname");
+  $fallback: mdc-theme-get-var-fallback_($style);
+
+  @return var(#{$var}, $fallback);
+}

packages/mdc-theme/_mixins.scss

@@ -23,6 +23,7 @@
 @import "@material/feature-targeting/functions";
 @import "@material/feature-targeting/mixins";
 @import "./variables";
+@import "./functions";
 
 @mixin mdc-theme-core-styles($query: mdc-feature-all()) {
   $feat-color: mdc-feature-create-target($query, color);
@@ -67,7 +68,17 @@
 // $edgeOptOut controls whether to feature-detect around Edge to avoid emitting CSS variables for it,
 // intended for use in cases where interactions with pseudo-element styles cause problems due to Edge bugs.
 @mixin mdc-theme-prop($property, $style, $important: false, $edgeOptOut: false) {
-  @if mdc-theme-is-valid-theme-prop-value_($style) {
+  @if mdc-theme-is-var-with-fallback_($style) {
+    @if $important {
+      #{$property}: mdc-theme-get-var-fallback_($style) !important;
+      /* @alternate */
+      #{$property}: mdc-theme-var_($style) !important;
+    } @else {
+      #{$property}: mdc-theme-get-var-fallback_($style);
+      /* @alternate */
+      #{$property}: mdc-theme-var_($style);
+    }
+  } @else if mdc-theme-is-valid-theme-prop-value_($style) {
     @if $important {
       #{$property}: $style !important;
     } @else {

packages/mdc-theme/_variables.scss

@@ -129,6 +129,10 @@ $mdc-theme-property-values: (
 //
 // NOTE: This function must be defined in _variables.scss instead of _functions.scss to avoid circular imports.
 @function mdc-theme-prop-value($style) {
+  @if mdc-theme-is-var-with-fallback_($style) {
+    @return mdc-theme-get-var-fallback_($style);
+  }
+
   @if mdc-theme-is-valid-theme-prop-value_($style) {
     @return $style;
   }