CircleButton.xaml

<UserControl x:Class="WPFMaterialDesign.Controls.CircleButton"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:WPFMaterialDesign.Controls"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300"
             DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <UserControl.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="CircleButtonStyle.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </UserControl.Resources>

    <Grid>
        <!--<Button Style="{StaticResource CircleButtonStyle}" Background="{Binding CircleBackgroundColor, Converter={StaticResource ColorBrushConverter}}"/>-->
        <Button Style="{StaticResource CircleButtonStyle}" Background="Red"/>
    </Grid>
</UserControl>