fix(slide-toggle): use default hue for thumb in dark theme (angular#1…

…4230)

* The thumb should use the `default` hue in dark themes to match the MDC behavior and also make it easier for people to create a custom theme without needing to explicitly specify the `200` hue just for the slide-toggle. This also makes the slide-toggle more consistent with the checkbox and radio (which are considered selection controls as well)

Fixes angular#14192

src/lib/slide-toggle/_slide-toggle-theme.scss

@@ -33,10 +33,12 @@
 
   // Color hues are based on the specs which briefly show the hues that are applied to a switch.
   // The 2018 specs no longer describe how dark switches should look like. Due to the lack of
-  // information for dark themed switches, we keep the old behavior that is based on the previous
-  // specifications. See: https://material.io/design/components/selection-controls.html#specs
+  // information for dark themed switches, we partially keep the old behavior that is based on
+  // the previous specifications. For the checked color we always use the `default` hue because
+  // that follows MDC and also makes it easier for people to create a custom theme without needing
+  // to specify each hue individually.
   $thumb-unchecked-hue: if($is-dark, 400, 50);
-  $thumb-checked-hue: if($is-dark, 200, default);
+  $thumb-checked-hue: default;
 
   $bar-unchecked-color: mat-color($foreground, disabled);
   $ripple-unchecked-color: mat-color($foreground, base);