You can remove the borders, like :
<TextBox Width="150" Height="40" >
<TextBox.Resources>
<Thickness x:Key="TextControlBorderThemeThicknessFocused">0,0,0,0</Thickness>
<Thickness x:Key="TextControlBorderThemeThickness">0,0,0,0</Thickness>
</TextBox.Resources>
</TextBox>