A ResourceDictionary can load one or more resource dictionaries using its MergedDictionaries property (ResourceDictionary.MergedDictionaries), which is a collection. It's not always mandatory to have a reference to other resource dictionaries, but it can also have its own resources as well:
<Application.Resources> <ResourceDictionary> <SolidColorBrush Color="Red" x:Key="redBrush" /> <SolidColorBrush Color="Green" x:Key="greenBrush" /> <SolidColorBrush Color="Blue" x:Key="blueBrush" /> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="Themes/SolidBrushes.xaml" /> <ResourceDictionary Source="Themes/GradientBrushes.xaml" /> <ResourceDictionary Source="Themes/Fonts.xaml" /> <...