Condition variables for dark mode to support of color-scheme

This ensures that native controls like file inputs will have proper text contrast, as some browsers will change the text of the button based on the colour set on the input, resulting in a white on light grey

packages/govuk-frontend/src/govuk/core/_theme.scss

@@ -25,16 +25,18 @@ $govuk-theme-from-system: true !default;
 }
 
 @mixin _govuk-theme-dark() {
-  color-scheme: dark;
+  @supports (color-scheme: dark) {
+    color-scheme: dark;
 
-  --govuk-canvas-background-colour: #333333;
-  --govuk-body-background-colour: #222222;
-  --govuk-text-colour: #ffffff;
-  --govuk-secondary-text-colour: #cccccc;
-  --govuk-link-colour: #add8e6;
-  --govuk-link-visited-colour: #ffb6c1;
-  --govuk-link-hover-colour: #00ffff;
-  --govuk-link-active-colour: #ffffff;
+    --govuk-canvas-background-colour: #333333;
+    --govuk-body-background-colour: #222222;
+    --govuk-text-colour: #ffffff;
+    --govuk-secondary-text-colour: #cccccc;
+    --govuk-link-colour: #add8e6;
+    --govuk-link-visited-colour: #ffb6c1;
+    --govuk-link-hover-colour: #00ffff;
+    --govuk-link-active-colour: #ffffff;
+  }
 }
 
 @include govuk-exports("govuk/core/theme") {