When an app is running, Display.isSystemDarkTheme is not reporting changes (GTK)

[nfalliere]

Describe the bug
The GTK implementation of Display.isSystemDarkTheme() is returning a boolean initialized once, when the Display is set up. If the theme is changed when the app is running, the value returned by isSystemDarkTheme() is stale and should be updated.

To Reproduce
Start a SWT app; check Display.isSystemDarkTheme(), which should match the current system theme; change the theme; check Display.isSystemDarkTheme(): nothing has changed.

Expected behavior
The GTK implementation of isSystemDarkTheme() should reflect changes in theme selection, as is the case on Windows.

Environment:

1. Select the platform(s) on which the behavior is seen:

• • Windows (OK on Windows)
• • Linux
• • macOS (unverified on macOS)

2. Additional OS info (e.g. OS version, Linux Desktop, etc)
   Tested on an up-to-date Ubuntu.

3. JRE/JDK version

4. JDK 17.

Version since
Tested with Eclipse 4.34 (December 2024).