<Swing Dev> RFR: 8260291: The case instruction is not visible in dark mode

[Pankaj Bansal]

Please review a trivial test only fix.

This is a manual test and create an instruction panel containing JTextArea. The JTextArea background color is hardcoded as white color, which is causing issues on dark mode in Ubuntu 20.04 and Ubuntu 20.10 as the text color is also white. The fix is to remove the hardcoded white color as the JTextArea background color should be set by L&F and should not be hardcoded in test.

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Commit messages:
 - Add bugID
 - 8260291: The case instruction is not visible in dark mode

Changes: https://git.openjdk.java.net/jdk/pull/2564/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2564&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260291
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2564.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2564/head:pull/2564

PR: https://git.openjdk.java.net/jdk/pull/2564