In your UserControl code, define the dependency properties for the brushes you want to use in your template.
public static readonly DependencyProperty ThumbnailBrushProperty = DependencyProperty.Register("Command", typeof (Brush), typeof (YourControl), new PropertyMetadata(default(Brush)));
This property will appear in the VS and Blend constructor in the Miscellaneous section.
If you understand correctly, you have created a ControlTemplate for your custom slider control. If so, you can use the property from the template by binding to it by binding the template:
UPDATE
Ok, so you are creating a template for a standard slider control. Have you considered the definition of brushes that you use as ThumbnailBrush etc. In a resource, refer to this resource dictionary in the resource dictionary that contains your slider template and just uses them from the template? In Blend, you could change it in the Resources tab.
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