In XAML
<editors:SfTextInputLayout
ContainerType="Outlined"
Hint="Unit No"
HintFloatMode="Float">
<editors:SfMaskedEdit
Name="username"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
HorizontalContentAlignment="Left"
Background="Transparent"
BorderThickness="0"
FontFamily="Helvetica"
FontSize="20"
FontWeight="Light"
Mask="(0[0-9]|1[0-9]|2[0-3])-[0-5][0-9]"
MaskType="RegEx" />
</editors:SfTextInputLayout>
in cs
SfSkinManager.SetTheme(this, new Theme("MaterialLight"));
MaterialLightThemeSettings themeSettings2 = new MaterialLightThemeSettings();
themeSettings2.Palette = Syncfusion.Themes.MaterialLight.WPF.MaterialPalette.Orange;
SfSkinManager.RegisterThemeSettings("MaterialLight", themeSettings2);
Note: Removing the above settings the transparent behaviour works