DataService data storage
Style  

It won't hurt you and makes your code less error prone, if you would use braces {} also for single if statements.  

Commented code (// if (isoSettings.Contains(propertyName)) isoSettings.Remove(propertyName);) should be removed completely. You should instead use a code versioning system like svn or git.  

Update based on the comments below
It is not about a comment but about commented out code which is just dead code. What value has dead code ? Does it make your live code more readable or does it make your live code easier to understand ? No, it is quite the opposite.  

Asume the following  

public static void SetProperty(string propertyName, object content)
{
    //if (System.ComponentModel.DesignerProperties.IsInDesignTool)
    //    return;

    //if (content == null)
        RemoveProperty(propertyName); // if (isoSettings.Contains(propertyName)) isoSettings.Remove(propertyName);

    isoSettings[propertyName] = content;
    isoSettings.Save();
}  


this is quite hard to read and understatnd by Mr.Maintainer.  

This instead  

public static void SetProperty(string propertyName, object content)
{
    RemoveProperty(propertyName);

    isoSettings[propertyName] = content;
    isoSettings.Save();
}


is much better to understand.  

If you check something into svn/git you always should add a descriptive message to the commit.  

SetProperty() method  


if (content == null)
        RemoveProperty(propertyName); // if (isoSettings.Contains(propertyName)) isoSettings.Remove(propertyName);

    isoSettings[propertyName] = content;
    isoSettings.Save();  



this does not make sense to me. If content==null you first remove the property, and then you add it. Next time passing null you just do the same.  

You should consider returning after you have removed the property. There is no need to save the setting.  

SetProperty() returning setted value  

This is in my opinion a bad idea for a method which seems to be like a property setter. A property setter should only set a properties value.