Clearing map canvas theme falls back to project layers

src/gui/qgsmapcanvas.cpp

@@ -1648,6 +1648,7 @@ void QgsMapCanvas::setTheme( const QString &theme )
   {
     mTheme.clear();
     mSettings.setLayerStyleOverrides( QMap< QString, QString>() );
+    setLayers( QgsProject::instance()->mapThemeCollection()->masterVisibleLayers() );
     emit themeChanged( QString() );
   }
   else

src/gui/qgsmapcanvas.h

@@ -323,9 +323,7 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
      * QgsLayerTreeMapCanvasBridge.
      *
      * If an empty string is passed then the current theme association will be
-     * cleared. The layers from the previously associated theme will remain
-     * in the canvas, and a call to setLayers() may be necessary to define
-     * which layers should be shown in the canvas.
+     * cleared.
      * @note added in QGIS 3.0
      * @see theme()
      */

tests/src/python/test_qgsmapcanvas.py

@@ -314,6 +314,13 @@ def testMapTheme(self):
         canvas.waitWhileRendering()
         self.assertTrue(self.canvasImageCheck('theme4', 'theme4', canvas))
 
+        # clear theme
+        canvas.setTheme('')
+        canvas.refresh()
+        canvas.waitWhileRendering()
+        # should be different - we should now render project layers
+        self.assertFalse(self.canvasImageCheck('theme4', 'theme4', canvas))
+
     def canvasImageCheck(self, name, reference_image, canvas):
         self.report += "<h2>Render {}</h2>\n".format(name)
         temp_dir = QDir.tempPath() + '/'