Persist the "isDark" property in preference store

This is needed when the user had previously selected a theme which is
not available anymore. In this case we want to fall back to respect
whether that previous choice was dark or not.

Author: BeckerWdf

bundles/org.eclipse.e4.ui.css.swt.theme/src/org/eclipse/e4/ui/css/swt/internal/theme/ThemeEngine.java

@@ -81,6 +81,7 @@ public class ThemeEngine implements IThemeEngine {
 	private HashMap<String, List<IResourceLocator>> sourceLocators = new HashMap<>();
 
 	private static final String THEMEID_KEY = "themeid";
+	private static final String THEME_IS_DARK_KEY = "themeIsDark";
 
 	public static final String THEME_PLUGIN_ID = "org.eclipse.e4.ui.css.swt.theme";
 
@@ -501,6 +502,7 @@ public void setTheme(ITheme theme, boolean restore, boolean force) {
 			EclipsePreferencesHelper.setCurrentThemeId(theme.getId());
 
 			pref.put(THEMEID_KEY, theme.getId());
+			pref.putBoolean(THEME_IS_DARK_KEY, theme.isDark());
 			try {
 				pref.flush();
 			} catch (BackingStoreException e) {
@@ -565,6 +567,10 @@ private String getPreferenceThemeId() {
 		return getPreferences().get(THEMEID_KEY, null);
 	}
 
+	private boolean getPreferenceThemeIsDark() {
+		return getPreferences().getBoolean(THEME_IS_DARK_KEY, false);
+	}
+
 	private IEclipsePreferences getPreferences() {
 		return InstanceScope.INSTANCE.getNode(FrameworkUtil.getBundle(ThemeEngine.class).getSymbolicName());
 	}
@@ -612,7 +618,7 @@ public void restore(String alternateTheme) {
 				 * this case want to fall back to respect whether that previous choice was dark
 				 * or not. https://github.com/eclipse-platform/eclipse.platform.ui/issues/2776
 				 */
-				overrideWithDarkTheme = prefThemeId.contains("dark");
+				overrideWithDarkTheme = getPreferenceThemeIsDark();
 			} else {
 				/*
 				 * No previous theme selection in preferences. In this case check if the system