docs: add info about custom elevation color johannesjo#4

Author: johannesjo

README.md

@@ -78,6 +78,9 @@ There are also several [utility functions and mixins](https://github.com/johanne
   @include mat-css-bg(300);
 }
 ```
+
+There are also [some additional hacks](additional-hacks.md) (e.g. adding a color to the elevation shadow) available in case you need them.
+
 ## Initialization Options
 You can provide different options before initialization to change the body class used for the dark theme and to provide different default styles:
 ```scss

additional-hacks.md

@@ -0,0 +1,40 @@
+## Custom elevation shadow color
+In case you also want to add a color to your shadows, you can include the following mixin directly before you execute `@include initMaterialCssVars();`. Thanks goes to @picc09 for finding this out.
+
+```scss
+$mat-css-palette-foreground: map-merge(
+  // if you don't want to enter ALL the properties
+  $mat-css-palette-foreground,
+  (
+      elevation: var(--your-elevation-color-variable)
+  // ...
+  )
+);
+
+@mixin mat-elevation($zValue, $color: $mat-elevation-color, $opacity: $mat-elevation-opacity) {
+  @if type-of($zValue) != number or not unitless($zValue) {
+    @error '$zValue must be a unitless number';
+  }
+  @if $zValue < 0 or $zValue > 24 {
+    @error '$zValue must be between 0 and 24';
+  }
+
+  $color-umbra: $color;
+  $color-penumbra: $color;
+  $color-ambient: $color;
+  @if type-of($color) != color {
+    $color-umbra: rgba($color, $opacity * 0.2);
+    $color-penumbra: rgba($color, $opacity * 0.14);
+    $color-ambient: rgba($color, $opacity * 0.12);
+  }
+
+  box-shadow: #{map-get(_get-umbra-map($color-umbra, $opacity), $zValue)},
+  #{map-get(_get-penumbra-map($color-penumbra, $opacity), $zValue)},
+  #{map-get(_get-ambient-map($color-ambient, $opacity), $zValue)};
+}
+
+// finally initialize
+@include initMaterialCssVars();
+```
+
+In this way you can change `elevation: black` into variables file.