Update tool button disabled style

PixiEditor/Styles/ThemeStyle.xaml

@@ -107,6 +107,11 @@
                     <Border Name="brd" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}">
                         <ContentPresenter/>
                     </Border>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter Property="Opacity" Value="0.5"/>
+                        </Trigger>
+                    </ControlTemplate.Triggers>
                 </ControlTemplate>
             </Setter.Value>
         </Setter>