Try something like this:
public void LoadStyle(string fileName)
{
if (File.Exists(fileName))
{
try
{
using (FileStream fileStream = new FileStream(fileName, FileMode.Open,
FileAccess.Read, FileShare.Read))
{
ResourceDictionary resourceDictionary = (ResourceDictionary)XamlReader.
Load(fileStream);
Resources.MergedDictionaries.Clear(); // optional
Resources.MergedDictionaries.Add(resourceDictionary);
}
}
catch { }
}
}