Expose dark mode flag to plugin authors

Summary:
Should be available now with
```
getFlipperLib().settings().isDarkMode
```

Differential Revision: D46685851

fbshipit-source-id: 0777437b7ca06051cc3fd4dc1b794242f453f2b5

desktop/flipper-frontend-core/src/devices/__tests__/BaseDevice.node.tsx

@@ -264,6 +264,7 @@ test('log listeners are resumed and suspended automatically - 1', async () => {
     enableMenuEntries: jest.fn(),
     selectPlugin: jest.fn(),
     showNotification: jest.fn(),
+    settings: jest.fn(),
   });
 
   device.loadDevicePlugins(
@@ -352,6 +353,7 @@ test('log listeners are resumed and suspended automatically - 2', async () => {
     enableMenuEntries: jest.fn(),
     selectPlugin: jest.fn(),
     showNotification: jest.fn(),
+    settings: jest.fn(),
   });
 
   device.loadDevicePlugins(

desktop/flipper-frontend-core/src/flipperLibImplementation/index.tsx

@@ -31,7 +31,10 @@ export function baseFlipperLibImplementation(
   | 'showNotification'
   | 'createPaste'
   | 'intern'
-> & {intern: Omit<FlipperLib['intern'], 'currentUser' | 'isConnected'>} {
+  | 'settings'
+> & {
+  intern: Omit<FlipperLib['intern'], 'currentUser' | 'isConnected'>;
+} {
   return {
     isFB: !constants.IS_PUBLIC_BUILD,
     logger,

desktop/flipper-plugin-core/src/plugin/FlipperLib.tsx

@@ -207,6 +207,9 @@ export interface FlipperLib {
   };
   remoteServerContext: RemoteServerContext;
   intern: InternAPI;
+  settings: () => {
+    isDarkMode: boolean;
+  };
 }
 
 interface InternAPI {

desktop/flipper-plugin-core/src/test-utils/test-utils.tsx

@@ -106,6 +106,7 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
       },
       downloadFile: createStubFunction(),
     },
+    settings: createStubFunction(),
   };
 }
 

desktop/flipper-server-companion/src/initializeFlipperLibImplementation.tsx

@@ -38,5 +38,8 @@ export function initializeFlipperLibImplementation(
       // TODO: Write to stdout/stderr?
       throw new Error('Not supported in headless context');
     },
+    settings() {
+      return {isDarkMode: false};
+    },
   });
 }

desktop/flipper-ui-core/src/utils/flipperLibImplementation/index.tsx

@@ -59,5 +59,16 @@ export function initializeFlipperLibImplementation(
       );
     },
     DetailsSidebarImplementation: DetailSidebarImpl,
+    settings() {
+      const darkModeState = store.getState().settingsState.darkMode;
+      let isDarkMode = darkModeState === 'dark';
+      if (
+        darkModeState === 'system' &&
+        window.matchMedia('(prefers-color-scheme:dark)').matches
+      ) {
+        isDarkMode = true;
+      }
+      return {isDarkMode};
+    },
   });
 }