App.xaml

<Application x:Class="CurrencyConvertor.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:CurrencyConvertor"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <Style x:Key="ButtonRound" TargetType="Button">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="Button">
                        <Border CornerRadius="5" Background="{TemplateBinding Background}" 
                        BorderThickness="0.5">
                            <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Application.Resources>
</Application>