Вам нужно переопределить шаблон по умолчанию.Добавьте следующий стиль к своим ресурсам App.xaml в качестве отправной точки: -
<Style x:Key="BorderlessReadOnlyCheckBox" TargetType="CheckBox">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="Foreground" Value="#FF000000"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Top"/>
<Setter Property="Padding" Value="4,1,0,0"/>
<Setter Property="IsEnabled" Value="False" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="CheckBox">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="16"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Checked">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="CheckIcon" Storyboard.TargetProperty="(UIElement.Opacity)" Duration="0" To="1"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Unchecked"/>
<VisualState x:Name="Indeterminate"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid HorizontalAlignment="Left" VerticalAlignment="Top" Width="16" Height="16">
<Path x:Name="CheckIcon" Margin="1,1,0,1.5" Fill="Green" Stretch="Fill" Opacity="0" Width="10.5" Height="10" Data="M102.03442,598.79645 L105.22962,597.78918 L106.78825,600.42358 C106.78825,600.42358 108.51028,595.74304 110.21724,593.60419 C112.00967,591.35822 114.89314,591.42316 114.89314,591.42316 C114.89314,591.42316 112.67844,593.42645 111.93174,594.44464 C110.7449,596.06293 107.15683,604.13837 107.15683,604.13837 z" FlowDirection="LeftToRight"/>
</Grid>
<ContentPresenter
Grid.Column="1"
x:Name="contentPresenter"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
Margin="{TemplateBinding Padding}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Теперь вы устанавливаете свой флажок: -
cbstatus = new CheckBox();
cbstatus.Style = (Style)Application.Current.Resources["BorderlessReadOnlyCheckBox"];