RFR: 8343123: Nimbus: javax/swing/JInternalFrame/bug6726866.java does not have green undecorated window

[Prasanta Sadhukhan]

Test fails in NimbusL&F as DesktopPane does not have green background as set by the user, which is because, the auto-generated [DesktopPanePainter class](https://github.com/openjdk/jdk/blob/eb40a88f4076360708402454a494907e8c0c845d/src/java.desktop/share/classes/javax/swing/plaf/nimbus/SynthPainterImpl.java#L100-L102) which has pre-coded color values does not honour user-defined color.

Fix is to make sure user-defined DesktopPane background color is honoured and is painted and only auto-generated class for L&F derived colors.
CI is ok..

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Commit messages:
 - 8343123: Nimbus: javax/swing/JInternalFrame/bug6726866.java does not have green undecorated window

Changes: https://git.openjdk.org/jdk/pull/22057/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22057&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343123
  Stats: 8 lines in 2 files changed: 5 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22057.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22057/head:pull/22057

PR: https://git.openjdk.org/jdk/pull/22057