<Swing Dev> RFR: 8058704: Nimbus does not honor JTextPane background color

[Prasanta Sadhukhan]

The Nimbus look and feel ignores the configured background color of a JTextPane and always uses white. 
Every other look and feel tested (Metal, Motif, and Windows) correctly honors the configured background color of a JTextPane.
Issue seems to be in the hardcoded background color in nimbus skin which is rectified via the fix. 
After this fix, the background color is correctly updated to whatever is being set via JTextPane.setBackground().

CI all test run is green.

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Commit messages:
 - Fix
 - Fix
 - Fix
 - 8058704: Nimbus does not honor JTextPane background color

Changes: https://git.openjdk.java.net/jdk/pull/4930/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4930&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8058704
  Stats: 84 lines in 2 files changed: 77 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4930.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4930/head:pull/4930

PR: https://git.openjdk.java.net/jdk/pull/4930