[PE] make color picker “dark theme” aware (#8109)

`java.awt` colors are not dark-theme friendly but `JBColor`s are. As a
bonus we can use the `Gray` constant.

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Author: pq

flutter-studio/src/io/flutter/editor/AndroidStudioColorPickerProvider.java

@@ -5,6 +5,7 @@
  */
 package io.flutter.editor;
 
+import com.intellij.ui.Gray;
 import com.intellij.ui.colorpicker.ColorPickerBuilder;
 import com.intellij.openapi.ui.popup.Balloon;
 import com.intellij.openapi.ui.popup.BalloonBuilder;
@@ -38,7 +39,7 @@ public void show(
     popup = null;
 
     final LightCalloutPopup colorPanel = new ColorPickerBuilder()
-      .setOriginalColor(initialColor != null ? initialColor : new Color(255, 255, 255))
+      .setOriginalColor(initialColor != null ? initialColor : Gray._255)
       .addSaturationBrightnessComponent()
       .addColorAdjustPanel()
       .addColorValuePanel().withFocus()