The easiest way to create a style is to copy an existing one and edit it.
Create a simple window with two buttons:
<Window x:Class="WPF_Style_Example.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" ResizeMode="NoResize"
Title="MainWindow"
Height="150" Width="200">
<Grid>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Button Margin="5" Content="Button 1"/>
<Button Margin="5" Grid.Row="1" Content="Button 2"/>
</Grid>
In Visual Studio, copying can be done by right-clicking on the first button in the editor and choosing "Edit a Copy..." under the "Edit Template" menu.
Define in "Application".
The following example shows a modified template to create an ellipse button:
<Style x:Key="ButtonStyle1" TargetType="{x:Type Button}">
<Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
<Setter Property="Background" Value="{StaticResource Button.Static.Background}"/>
<Setter Property="BorderBrush" Value="{StaticResource Button.Static.Border}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Grid>
<Ellipse x:Name="ellipse" StrokeThickness="{TemplateBinding BorderThickness}" Stroke="{TemplateBinding BorderBrush}" Fill="{TemplateBinding Background}" SnapsToDevicePixels="true"/>
<ContentPresenter x:Name="contentPresenter" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsDefaulted" Value="true">
<Setter Property="Stroke" TargetName="ellipse" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Fill" TargetName="ellipse" Value="{StaticResource Button.MouseOver.Background}"/>
<Setter Property="Stroke" TargetName="ellipse" Value="{StaticResource Button.MouseOver.Border}"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Fill" TargetName="ellipse" Value="{StaticResource Button.Pressed.Background}"/>
<Setter Property="Stroke" TargetName="ellipse" Value="{StaticResource Button.Pressed.Border}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Fill" TargetName="ellipse" Value="{StaticResource Button.Disabled.Background}"/>
<Setter Property="Stroke" TargetName="ellipse" Value="{StaticResource Button.Disabled.Border}"/>
<Setter Property="TextElement.Foreground" TargetName="contentPresenter" Value="{StaticResource Button.Disabled.Foreground}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
The result: