The case instruction is not visible in dark mode

Testsuite: Regression manual
Test name(s):javax/swing/JSpinner/TestJSpinnerPressUnpress.java
Product(s) tested: JDK 16b28 (64bit)
OS/architecture: ubuntu20.10-x64(dark Mode)/Jtreg5.1b01
option:-Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel

Reproducible: Always
Reproducible on machine: Evergreen-machine
[If Sometimes] Also reproducible on machine:

Actual behavior:
1) The case description cannot be seen because the background color of the case description is the same as the foreground color, please refer to PressUnpress-dark.png
2) There is no such issue with light and standard mode, please refer PressUnpress-dark-light-standard.png
3) On ubuntu20.04(dark mode) with JDK16b28:It's failed,there has the same issue
4) On ubuntu20.10-x64(Dark Mode) with the same option,JDK11.0.10b08&ws/JDK15.0.1b09&ws/JDK15.0.2b07&ws failed as the same issue.
5) There is no such test on JDK7u291b09&ws/JDK8u281b09&ws

Affect Test:
javax/swing/text/Caret/8163124/CaretFloatingPointAPITest.java
javax/swing/plaf/synth/SynthCheckBoxMenuItem/TestJCheckBoxMenuItem.java
javax/swing/plaf/metal/MetalIcons/MetalHiDPIIconsTest.java
javax/swing/RepaintManager/8162350/RepaintManagerFPUIScaleTest.java
javax/swing/JTextPane/TestJTextPaneHTMLRendering.java