<ControlTemplate x:Key="MyTemplate" TargetType="Button">
                <ControlTemplate.Triggers>
                    <Trigger>
                        <Trigger.Property>IsMouseOver</Trigger.Property>
                        <Trigger.Value>True</Trigger.Value>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate >

 Trigger trig = new Trigger() { Property = UIElement.IsMouseOverProperty, Value = true };
            Setter set = new Setter() { Property = Control.FontStyleProperty, Value = FontStyles.Italic };
            trig.Setters.Add(set);
            template.Triggers.Add(trig);
            yellowbut.Template = template;
 

<Button Style="{StaticResource MyStyle}" Click="Button_Click" Name="yellowbut">press me</Button>