Setting and restoring a default style

To set a layer's current style as the default style for use in other QGIS projects, go to Style | Save As Default in the Layer Properties window. This will save a .qml file with the same name as the layer on disk. When the layer is added to the map canvas (in this or another QGIS project), the saved default style will be automatically applied.

If you have made changes to the style of a layer that had a default style and wish to revert back to the default style, go to Style | Restore Default Style.

Lastly, to remove a default style for a layer, delete the .qml file of the same name as the layer on disk.