refactor: use transparent background for charts in aura (#10591)

Author: jouni

packages/aura/aura.css

@@ -11,6 +11,7 @@
 @import './src/components/avatar.css';
 @import './src/components/button.css';
 @import './src/components/card.css';
+@import './src/components/charts.css';
 @import './src/components/checkbox-radio.css';
 @import './src/components/confirm-dialog.css';
 @import './src/components/crud.css';

packages/aura/src/components/charts.css

@@ -0,0 +1,4 @@
+:where(:root),
+:where(:host) {
+  --vaadin-charts-background: transparent;
+}