RFR: 8355012: JavaFX modena.css -fx-highlight-text-fill bug

[Ziad El Midaoui]

The issue was happening because `-fx-highlight-text-fill` was set to white , so when the background color is bright (>60% brightness ) the text is not visible , the solution is to change the value to be dynamically set depending on the level of brightness of the background this is done by `-fx-focused-text-base-color` .

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Commit messages:
 - Changed value of -fx-highlight-text-fill to be dynamically set and added tests

Changes: https://git.openjdk.org/jfx/pull/1801/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1801&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355012
  Stats: 34 lines in 2 files changed: 33 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1801.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1801/head:pull/1801

PR: https://git.openjdk.org/jfx/pull/1801