diff --git a/src/app/plugins/call/CallEmbed.ts b/src/app/plugins/call/CallEmbed.ts
index 7d87b4109..f029bfaec 100644
--- a/src/app/plugins/call/CallEmbed.ts
+++ b/src/app/plugins/call/CallEmbed.ts
@@ -133,6 +133,15 @@ export class CallEmbed {
 
   private themeKind: ElementCallThemeKind = 'dark';
 
+  // The theme the frame was loaded with (its URL `theme` param), and whether
+  // the widget API is ready. Until it is, the iframe may still be on its
+  // initial about:blank (our own origin), so a send to the call's origin is
+  // refused ("target origin … does not match the recipient window's origin")
+  // and the request hangs to a timeout. Theme changes wait for ready.
+  private loadedThemeKind: ElementCallThemeKind = 'dark';
+
+  private widgetReady = false;
+
   // Watchdog: detects an iframe that never reaches a usable state.
   private loadWatchdog?: ReturnType<typeof setTimeout>;
 
@@ -360,6 +369,7 @@ export class CallEmbed {
     this.iframe = iframe;
     this.container = container;
     this.themeKind = themeKind;
+    this.loadedThemeKind = themeKind;
 
     const controlState = initialControlState ?? new CallControlState(true, false, true);
     this.control = new CallControl(controlState, call, iframe);
@@ -375,6 +385,13 @@ export class CallEmbed {
     // happy path (onCallJoined) clears it too; these cover earlier signals so a
     // user sitting in the lobby/prescreen isn't flagged as an error.
     this.disposables.push(this.onReady(() => this.settleLoad()));
+    this.disposables.push(
+      this.onReady(() => {
+        this.widgetReady = true;
+        // A theme change made while the frame was loading.
+        if (this.themeKind !== this.loadedThemeKind) this.sendTheme();
+      }),
+    );
     this.disposables.push(this.onCapabilitiesNotified(() => this.settleLoad()));
     this.disposables.push(this.onPreparingError(() => this.settleLoad('iframe')));
 
@@ -399,12 +416,18 @@ export class CallEmbed {
     return this.room.roomId;
   }
 
-  public setTheme(theme: ElementCallThemeKind) {
+  public setTheme(theme: ElementCallThemeKind): Promise<unknown> {
     this.themeKind = theme;
+    // Before ready, the URL theme applies and the change is sent on ready.
+    if (!this.widgetReady) return Promise.resolve();
+    return this.sendTheme();
+  }
+
+  private sendTheme(): Promise<unknown> {
     return this.call.transport
-      .send(WidgetApiToWidgetAction.ThemeChange, { name: theme })
+      .send(WidgetApiToWidgetAction.ThemeChange, { name: this.themeKind })
       .catch(() => {
-        // Widget transport not ready yet; theme URL param is the fallback
+        // The frame went away mid-send; nothing to do.
       });
   }