How to customize the properties of (outline border colors, hint name, helper text, error text, and char count) of TextInputLayout in Xamarin.Forms

Is it required to add the key? Yes. It is required, based on that only color has been applied.

How to make these styles get applied to some specific fields and not all SfTextInputLayout.

It has been achieved by adding the Resource with Syncfusion theme only for the desired SfTextInputLayout as per in below code snippet

[XAML]

 
	<inputLayout:SfTextInputLayout.Resources>             
	  <syncTheme:SyncfusionThemeDictionary>                 
		<syncTheme:SyncfusionThemeDictionary.MergedDictionaries>                     
			<ResourceDictionary>                         
				<x:String x:Key="SfTextInputLayoutTheme">CustomTheme</x:String>                         
				<!--Hint Color for focused state-->                         
				<Color x:Key="SyncPrimaryLightColor">Black</Color>                         
				<!--Hint Color for default state-->                         
				<Color x:Key="SfTextInputLayoutHintColor">Gray</Color>                         
				<!--Border Color for focused state-->                         
				<Color x:Key="SyncPrimaryColor">DarkCyan</Color>                         
				<!--Border Color for default state-->                         
				<Color x:Key="SfTextInputLayoutLineColor">Gray</Color>                         
				<!--Helper text Color for default state-->                         
				<Color x:Key="SfTextInputLayoutHelperTextColor">BlueViolet</Color>                         
				<!--Character count text Color for default state-->                         
				<Color x:Key="SfTextInputLayoutCounterLabelColor">DarkGoldenrod</Color>                         
				<!--Error text Color for default state-->                         
				<Color x:Key="SfTextInputLayoutErrorTextColor">Red</Color>                     
			</ResourceDictionary>                 
		</syncTheme:SyncfusionThemeDictionary.MergedDictionaries>             
	  </syncTheme:SyncfusionThemeDictionary>         
	</inputLayout:SfTextInputLayout.Resources> 

Reference sample