We were not able to reproduce the issue you describe in your post.
As you can read here, the DocumentInheritsDefaultStyleSettings is a way to set default values for the font family, font size, font style and font weight. However, changing the values from the UI should also work.
If the issue persists, please prepare a sample project illustrating the issue and attach it to a support ticket, so that we can look into the causes of the incorrect behavior.
the Telerik team