WPF/XAML: Moving resources from the app.xaml file and allowing global access all round

Okay, so I've created a few resource dictionaries in my application.

However, the app.xaml is getting really big so it's getting difficult to manage. I know how to create a new resource dictionary file. However, I'm struggling to find a way for the all of the resource dictionaries to access eachother's resources if I'm not placing them all in the same location.

Here's an example of some color resources I want to re-locate.

            <Color x:Key="ContentAreaColorLight">#686868</Color>
            <Color x:Key="ContentAreaColorDark">#1E1E1E</Color>

            <Color x:Key="DisabledControlLightColor">#FFE8EDF9</Color>
            <Color x:Key="DisabledControlDarkColor">#FFC5CBF9</Color>
            <Color x:Key="DisabledForegroundColor">#FF888888</Color>

Select allOpen in new window

Plus Styles ...

           <SolidColorBrush x:Key="ColourBackgroundWindowGeneral" Color="#333337" />
            <SolidColorBrush x:Key="ColourForegroundWindowGeneral" Color="#E6E6E6" />
            <SolidColorBrush x:Key="ColourGroupboxGeneral" Color="#569CD6" />
            <SolidColorBrush x:Key="ColourCheckboxGeneral" Color="#B99E65" />
            <SolidColorBrush x:Key="ColourButtonHoverGeneral" Color="#686868" />
            <SolidColorBrush x:Key="ColourButtonDisabledGeneral" Color="#686868" />
            <Style x:Key="StyleWindowGeneral" TargetType="Window">
                <Setter Property="Foreground" Value="{DynamicResource ResourceKey=ColourForegroundWindowGeneral}" />
                <Setter Property="Background" Value="{DynamicResource ResourceKey=ColourBackgroundWindowGeneral}" />
                <Setter Property="BorderThickness" Value="0" />
                <Setter Property="WindowStyle" Value="None" />
                <Setter Property="ResizeMode" Value="CanResizeWithGrip" />
                <Setter Property="AllowsTransparency" Value="True" />
                <Setter Property="ShowInTaskbar" Value="False" />
            </Style>

Select allOpen in new window

I imagine it's pretty simple but I don't seem to be able to find the answer. Maybe I'm using the wrong terminology?