feat(theming): allow request-scoped light/dark theme override

Support ?theme=light and ?theme=dark as a request-scoped override that is
not persisted to user settings and never bypasses an admin-configured
enforce_theme. Invalid, empty, non-string or unknown values are ignored,
and non-visual themes such as the OpenDyslexic font theme are preserved.
The override is applied only in ThemesService::getEnabledThemes() so that
getThemes() and theme registration remain unchanged.

Assisted-by: Cline
Signed-off-by: Jack Arru <giacomo@beta.srl>

Author: giacomoarru

apps/theming/lib/Service/ThemesService.php

@@ -16,17 +16,25 @@
 use OCA\Theming\Themes\HighContrastTheme;
 use OCA\Theming\Themes\LightTheme;
 use OCP\IConfig;
+use OCP\IRequest;
 use OCP\IUser;
 use OCP\IUserSession;
 use Psr\Log\LoggerInterface;
 
 class ThemesService {
+	private const REQUEST_THEME_PARAM = 'theme';
+	private const REQUEST_THEME_OVERRIDES = [
+		'light',
+		'dark',
+	];
+
 	/** @var ITheme[] */
 	private array $themesProviders;
 
 	public function __construct(
 		private IUserSession $userSession,
 		private IConfig $config,
+		private IRequest $request,
 		private LoggerInterface $logger,
 		private DefaultTheme $defaultTheme,
 		LightTheme $lightTheme,
@@ -161,6 +169,12 @@ public function getEnabledThemes() {
 			if ($enforcedTheme !== '') {
 				return [$enforcedTheme];
 			}
+
+			$requestThemeOverride = $this->getRequestThemeOverride();
+			if ($requestThemeOverride !== null) {
+				return [$requestThemeOverride];
+			}
+
 			return [];
 		}
 
@@ -171,12 +185,51 @@ public function getEnabledThemes() {
 		}
 
 		try {
-			return $enabledThemes;
+			return $this->applyRequestThemeOverride($enabledThemes);
 		} catch (\Exception $e) {
 			return [];
 		}
 	}
 
+	/**
+	 * Apply a request-scoped light/dark theme override without persisting it.
+	 *
+	 * @param list<string> $themes
+	 * @return list<string>
+	 */
+	private function applyRequestThemeOverride(array $themes): array {
+		$requestThemeOverride = $this->getRequestThemeOverride();
+		if ($requestThemeOverride === null) {
+			return $themes;
+		}
+
+		$theme = $this->themesProviders[$requestThemeOverride];
+		$themes = array_filter($themes, function (string $themeId) use ($theme): bool {
+			return !isset($this->themesProviders[$themeId])
+				|| $this->themesProviders[$themeId]->getType() !== $theme->getType();
+		});
+
+		return array_values(array_unique(array_merge($themes, [$requestThemeOverride])));
+	}
+
+	private function getRequestThemeOverride(): ?string {
+		$requestThemeOverride = $this->request->getParam(self::REQUEST_THEME_PARAM, '');
+		if (!is_string($requestThemeOverride)) {
+			return null;
+		}
+
+		$requestThemeOverride = strtolower(trim($requestThemeOverride));
+		if (!in_array($requestThemeOverride, self::REQUEST_THEME_OVERRIDES, true)) {
+			return null;
+		}
+
+		if (!isset($this->themesProviders[$requestThemeOverride])) {
+			return null;
+		}
+
+		return $requestThemeOverride;
+	}
+
 	/**
 	 * Set the list of enabled themes
 	 * for the logged-in user

apps/theming/tests/Service/ThemesServiceTest.php

@@ -23,6 +23,7 @@
 use OCP\App\IAppManager;
 use OCP\IConfig;
 use OCP\IL10N;
+use OCP\IRequest;
 use OCP\IURLGenerator;
 use OCP\IUser;
 use OCP\IUserSession;
@@ -33,6 +34,7 @@
 class ThemesServiceTest extends TestCase {
 	private IUserSession&MockObject $userSession;
 	private IConfig&MockObject $config;
+	private IRequest&MockObject $request;
 	private LoggerInterface&MockObject $logger;
 
 	private ThemingDefaults&MockObject $themingDefaults;
@@ -44,6 +46,7 @@ class ThemesServiceTest extends TestCase {
 	protected function setUp(): void {
 		$this->userSession = $this->createMock(IUserSession::class);
 		$this->config = $this->createMock(IConfig::class);
+		$this->request = $this->createMock(IRequest::class);
 		$this->logger = $this->createMock(LoggerInterface::class);
 		$this->themingDefaults = $this->createMock(ThemingDefaults::class);
 
@@ -60,6 +63,7 @@ protected function setUp(): void {
 		$this->themesService = new ThemesService(
 			$this->userSession,
 			$this->config,
+			$this->request,
 			$this->logger,
 			...array_values($this->themes)
 		);
@@ -232,6 +236,93 @@ public function testGetEnabledThemes(): void {
 		$this->assertEquals(['default'], $this->themesService->getEnabledThemes());
 	}
 
+	public function testGetEnabledThemesRequestThemeForGuest(): void {
+		$this->userSession->expects($this->any())
+			->method('getUser')
+			->willReturn(null);
+		$this->config->expects($this->once())
+			->method('getSystemValueString')
+			->with('enforce_theme', '')
+			->willReturn('');
+		$this->request->expects($this->once())
+			->method('getParam')
+			->with('theme', '')
+			->willReturn('dark');
+
+		$this->assertEquals(['dark'], $this->themesService->getEnabledThemes());
+	}
+
+	public function testGetEnabledThemesRequestThemeOverridesUserTheme(): void {
+		$user = $this->createMock(IUser::class);
+		$this->userSession->expects($this->any())
+			->method('getUser')
+			->willReturn($user);
+		$user->expects($this->any())
+			->method('getUID')
+			->willReturn('user');
+
+		$this->config->expects($this->once())
+			->method('getUserValue')
+			->with('user', Application::APP_ID, 'enabled-themes', '["default"]')
+			->willReturn(json_encode(['dark', 'opendyslexic']));
+		$this->config->expects($this->once())
+			->method('getSystemValueString')
+			->with('enforce_theme', '')
+			->willReturn('');
+		$this->request->expects($this->once())
+			->method('getParam')
+			->with('theme', '')
+			->willReturn('light');
+
+		$this->assertEquals(['opendyslexic', 'light'], $this->themesService->getEnabledThemes());
+	}
+
+	public function testGetEnabledThemesInvalidRequestTheme(): void {
+		$this->userSession->expects($this->any())
+			->method('getUser')
+			->willReturn(null);
+		$this->config->expects($this->once())
+			->method('getSystemValueString')
+			->with('enforce_theme', '')
+			->willReturn('');
+		$this->request->expects($this->once())
+			->method('getParam')
+			->with('theme', '')
+			->willReturn('sepia');
+
+		$this->assertEquals([], $this->themesService->getEnabledThemes());
+	}
+
+	public function testGetEnabledThemesNonStringRequestThemeIsIgnored(): void {
+		$this->userSession->expects($this->any())
+			->method('getUser')
+			->willReturn(null);
+		$this->config->expects($this->once())
+			->method('getSystemValueString')
+			->with('enforce_theme', '')
+			->willReturn('');
+		$this->request->expects($this->once())
+			->method('getParam')
+			->with('theme', '')
+			->willReturn(['dark']);
+
+		$this->assertEquals([], $this->themesService->getEnabledThemes());
+	}
+
+	public function testGetEnabledThemesRequestThemeDoesNotOverrideEnforcedTheme(): void {
+		$this->userSession->expects($this->any())
+			->method('getUser')
+			->willReturn(null);
+		$this->config->expects($this->once())
+			->method('getSystemValueString')
+			->with('enforce_theme', '')
+			->willReturn('light');
+		$this->request->expects($this->never())
+			->method('getParam');
+
+		$this->assertEquals(['light'], $this->themesService->getEnabledThemes());
+	}
+
 	public function testGetEnabledThemesEnforced(): void {
 		$user = $this->createMock(IUser::class);
 		$this->userSession->expects($this->any())