feat(editor): sync export theme with ui theme (#10903)

packages/excalidraw/components/App.tsx

@@ -595,6 +595,7 @@ const gesture: Gesture = {
 class App extends React.Component<AppProps, AppState> {
   canvas: AppClassProperties["canvas"];
   interactiveCanvas: AppClassProperties["interactiveCanvas"] = null;
+  public sessionExportThemeOverride: AppState["theme"] | undefined;
   rc: RoughCanvas;
   unmounted: boolean = false;
   actionManager: ActionManager;
@@ -710,6 +711,7 @@ class App extends React.Component<AppProps, AppState> {
     this.state = {
       ...defaultAppState,
       theme,
+      exportWithDarkMode: theme === THEME.DARK,
       isLoading: true,
       ...this.getCanvasOffsets(),
       viewModeEnabled,
@@ -3241,6 +3243,13 @@ class App extends React.Component<AppProps, AppState> {
     const elements = this.scene.getElementsIncludingDeleted();
     const elementsMap = this.scene.getElementsMapIncludingDeleted();
 
+    const shouldExportWithDarkMode =
+      (this.sessionExportThemeOverride ?? this.state.theme) === THEME.DARK;
+
+    if (this.state.exportWithDarkMode !== shouldExportWithDarkMode) {
+      this.setState({ exportWithDarkMode: shouldExportWithDarkMode });
+    }
+
     if (!this.state.showWelcomeScreen && !elements.length) {
       this.setState({ showWelcomeScreen: true });
     }