The UI you have now automatically applies the default style that is defined in the styles_def.xml file, shich is located in the res/raw folder. You can override this with custom settings in the styles_def.xml.
<Style TargetType="Label" Key="CustomStyle" BasedOn="DefLabel" platform="android">
<Setter Property="Foreground" Value="#F0AB00" />
<Setter Property="FontFamily" Value="sans_serif" />
<Setter Property="FontSize" Value="16" />
</Style>
<UIElement type="label">
<!-- Assign the text to be displayed -->
<P pid="text" value="Hello World! This is Extensibility!" />
<P pid="style" value="CustomStyle" />
</UIElement>