Retrieving the theme via org.eclipse.e4.ui.css.swt.theme does not always work

[jomillerOpen]

If the workspace's theme is inherrited from the default value (say it's been set to dark mode).
If I try to get the current workspace's theme via:
(Code samples simplified)
Preferences preferences1 = InstanceScope.INSTANCE.getNode("org.eclipse.e4.ui.css.swt.theme");
String themeCssUri2 = preferences1.get("themeid", "");
Preferences preferences2 = ConfigurationScope.INSTANCE.getNode("org.eclipse.e4.ui.css.swt.theme");
String themeCssUri3 = preferences2.get("themeid", "");
Preferences preferences3 = DefaultScope.INSTANCE.getNode("org.eclipse.e4.ui.css.swt.theme");
String themeCssUri4 = preferences3.get("themeid", "");

or via

IPreferencesService service = Platform.getPreferencesService();
String themeId = service.getString("org.eclipse.e4.ui.css.swt.theme", "currentThemeId", null, null);
String themeId2 = service.getString("org.eclipse.e4.ui.css.swt.theme", "currentThemeId", null, new IScopeContext[] { InstanceScope.INSTANCE });
String themeId3 = service.getString("org.eclipse.e4.ui.css.swt.theme", "currentThemeId", null, new IScopeContext[] { ConfigurationScope.INSTANCE });
String themeId4 = service.getString("org.eclipse.e4.ui.css.swt.theme", "currentThemeId", null, new IScopeContext[] {DefaultScope.INSTANCE });

The themeID is always blank. If the preference was set on the workspace level (so that there's a preference file located at .metadata.plugins\org.eclipse.core.runtime\org.eclipse.e4.ui.css.swt.theme.prefs, then the themeID can be retrieved.

The only way I can currently reliably retrieve the themeID with and without the workspace preference file present, is to use:
IThemeEngine engine = PlatformUI.getWorkbench().getService(IThemeEngine.class);
ITheme activeTheme = engine.getActiveTheme();
String themeCssUri = activeTheme.getId();

When I was using copilot to help troubleshoot, it seemed to think that the e4 theme service injected the theme on load time, so it was bypassing the regular settings mechanisms.

Steps to reproduce

Have the default theme set to dark mode. Open a workspace where the file .metadata.plugins\org.eclipse.core.runtime\org.eclipse.e4.ui.css.swt.theme.prefs is not present. Try the above methods to retrieve the current theme's string (should be "org.eclipse.e4.ui.css.theme.e4_dark").

I expected: That the themeID, like other preference settings, would be retrievable at one of the scopes available at the very least. I can understand that without the preference file in the workspace that it wouldn't be available at the workspace scope, but one of the higher scopes should still have the value available.

Tested under this environment:

• OS & version: Windows 11
• Eclipse IDE/Platform version (as shown in Help > About): Eclipse 2026-06, org.eclipse.e4.ui.css.swt.theme=0.15.100v20260507-0340

Community

• [ Y] I understand reporting an issue to this OSS project does not mandate anyone to fix it. Other contributors may consider the issue, or not, at their own convenience. The most efficient way to get it fixed is that I fix it myself and contribute it back as a good quality patch to the project.

[vogella]

Like this:

MApplication application = workbench.getService(MApplication.class);
IEclipseContext context = application.getContext();
IThemeEngine engine = context.get(IThemeEngine.class);
ITheme active = engine.getActiveTheme();   // may be null if none set

Alternatively if you want to use the preferences this should also work (not tested):

import org.eclipse.core.runtime.Platform;
import org.eclipse.core.runtime.preferences.IPreferencesService;

IPreferencesService service = Platform.getPreferencesService();
// searches the default scope order (instance → config → default) and returns null if unset
String themeId = service.getString("org.eclipse.e4.ui.css.swt.theme", "themeid", null, null);

If if you only want the defaut, use: IEclipsePreferences themeNode = UserScope.INSTANCE.getNode("org.eclipse.e4.ui.css.swt.theme"); as I did in 1baf984

Hope this helps.

[iloveeclipse]

Alternatively if you want to use the preferences this should also work (not tested):

I believe exactly this is the point here, which is not working, see microsoft/copilot-for-eclipse#329 (comment)

The code in question had no dependencies to org.eclipse.e4.ui.css.swt.theme (no need for any css related API except checking which theme - dark or light is set), and checking via preferences doesn't work.

[vogella]

Maybe the preferenceService has not been reworked to consider the user preferences? eclipse-equinox/equinox#446 introduced them, not sure if Platform.getPreferencesService() was updated to support them.

[iloveeclipse]

Maybe the preferenceService has not been reworked to consider the user preferences

Not sure how it should be related to user preferences, preference store should return which theme is set, but current theme, if user never changed preferences, is never set in preferences store and can be only queried via API. Once the theme is explicitly set by user, it can be queried by preferences store.

[jomillerOpen]

According to Opus 4.8:

Here's a consolidated summary of the investigation.
Root cause
When the theme is inherited from the OS (dark OS, no workspace .prefs file), the theme id is applied in memory only and is never written to any preference scope. That's why IThemeEngine.getActiveTheme().getId() works but every preference-based read returns blank.
The code path (all in ThemeEngine.java)

1. At startup, PartRenderingEngine.setCSSTheme() (lines 1412–1418) calls themeEngine.restore(cssTheme) for the normal (non–high-contrast) case.
2. restore(String) (lines 591–630):
   • Reads getPreferenceThemeId() → Platform.getPreferencesService().getString("org.eclipse.e4.ui.css.swt.theme", "themeid", null, null).
   • If a stored value exists → setTheme(t, false).
   • If not, and OS is dark → computes Display.isSystemDarkTheme(), then setTheme(E4_DARK_THEME_ID, false).
   • Both paths pass restore = false.
3. setTheme(theme, restore, force) (lines 447–522):
   • Sets this.currentTheme = theme (in memory → this is what getActiveTheme() returns).
   • The only persistence code is guarded by if (restore):

if (restore) { // SKIPPED when restore == false
IEclipsePreferences pref = getPreferences(); // InstanceScope node
pref.put(THEMEID_KEY, theme.getId());
pref.flush();
}

Since restore == false, themeid is never written to InstanceScope or anywhere else.
The restore = false is intentional: it keeps "follow the OS appearance" dynamic, so switching the OS theme is respected on the next launch.
Why each retrieval approach fails
Approach Result Reason
"themeid" @ Instance/Config/Default scope blank Correct key (THEMEID_KEY = "themeid"), but the value is never persisted when inherited.
"currentThemeId" @ any scope blank Wrong key. currentThemeId is only an in-memory static field in EclipsePreferencesHelper (line 37); it's never stored.
DefaultScope for "themeid" blank No AbstractPreferenceInitializer / plugin_customization.ini default exists; the "default = dark" decision is computed at runtime from Display.isSystemDarkTheme().
IThemeEngine.getActiveTheme().getId() works Returns the in-memory currentTheme set by setTheme().
How the Preferences page shows "Dark"
ViewsPreferencePage (General > Appearance) does not read the preference node — it reads the live engine:
• init(): engine = context.get(IThemeEngine.class) (line 460).
• createContents(): populates the combo from engine.getThemes() (line 170) and pre-selects engine.getActiveTheme() (lines 172–174).
• performOk(): calls engine.setTheme(theme, true) (line 470, restore=true) — so simply opening the page and clicking Apply/OK materializes the inherited value into the workspace .prefs file, after which themeid reads start working. This likely explains why some workspaces have the file and others don't.
DefaultScope persistence behavior (equinox source)
DefaultPreferences.flush() is a no-op:

// DefaultPreferences.java (lines 251-254)
@OverRide
public void flush() {
// default values are not persisted
}

The default scope is rebuilt each session via load() (lines 284–296). So writing to DefaultScope at runtime creates no file and is purely in-memory for the session.
Recommendation
• For your own code now: use IThemeEngine.getActiveTheme().getId() — the supported, always-correct API including inherited values.
• For a platform fix: in ThemeEngine.restore(), after computing themeToRestore, also set it in DefaultScope.INSTANCE.getNode(THEME_PLUGIN_ID) (no flush) so it's discoverable via the preferences API without writing a file or breaking OS-following. Caveats:
• Don't write to InstanceScope + flush() (that persists a file and freezes the choice).
• Change the explicit-choice detection in restore() to read InstanceScope explicitly (its current null-context lookup includes DefaultScope, so a populated default could be misread as an explicit user selection).
• Set the same id passed to setTheme(...) so getActiveTheme() and the preference can't diverge.

[vogella]

Hard to read what you pasted, but sounds like the analysis is wrong.

If the user scoped theme is used we intentially do not modify the instance scope. This is the same behavior for configuration or default scope, configuration scope values are also not copied into instance scope.

[vogella]

I suggest checking if Platform.getPreferencesService() returns also user scope values and if not, fixing it. The default theme is not stored as instance theme, the instance theme is allowed to override the user scoped theme.