WPF - Why doesn't Visual Studio find my Style in separate ResourceDictionary?

I created and added the following ResourceDictionary within my App.Resources

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns:local="clr-namespace:GanttTesting.Views"
                xmlns:sys="clr-namespace:System;assembly=mscorlib">

<sys:Double x:Key="CommandCenterExpandedTagRatio">1</sys:Double>
<sys:Double x:Key="CommandCenterCollapsedTagRatio">0</sys:Double>
<sys:String x:Key="CommandCenterCollapsedTagRatioAsString">0.1</sys:String>
<sys:Double x:Key="CommandCenterExpandedWidth">330</sys:Double>

<Style x:Key="CommandCenterStyle" TargetType="{x:Type local:CommandCenter}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate>
                <Grid Background="Red" Width="100" Height="100"/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

However, when using the "Create new Binding" window in Visual Studio and selecting "Static Resource", while "CommandCenterExpandedTagRatio" and the other String and Double values are displayed properly, "CommandCenterStyle" doesn't appear. Typing it manually doesn't work either.
Did I do something wrong here? Thanks a lot for your help!