You need to override the ControlTemplate of the Button:
<Button Content="save" Name="btnSaveEditedText"
Background="Transparent"
Foreground="White"
FontFamily="Tw Cen MT Condensed"
FontSize="30"
Margin="-280,0,0,10"
Width="60"
BorderBrush="Transparent"
BorderThickness="0">
<Button.Template>
<ControlTemplate TargetType="Button">
<ContentPresenter Content="{TemplateBinding Content}"/>
</ControlTemplate>
</Button.Template>
</Button>