How to create a style based on default style?

I would recommand to have a look at : https://justinmchase.com/2009/05/29/derived-styles-based-on-unnamed-default-styles/ It would go like this for you :

<Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}">

Pretty much the same. Just minus the x:Type with more explicit naming.

<Style TargetType="TextBox" BasedOn="{StaticResource DefaultTextBoxStyle}">

More information here in the docs. PS, in case you need the default templates, TextBox for example would normally be found in CoreStyles.xaml

ADDENDUM as requested in the comments in case you're confused at the first read of the answer;

"you DO need a base style, which is really easy to do as you're meant to do it in an application theme like silverlight provides by default (wpf/uwp etc won't have these) that creates the files like ToolkitStyles.xaml, SDKStyles.xaml, CoreStyles.xaml, etc... Which is WHERE the staticresource name in the answer came from as targeting a silverlight version from the year this was originally answered."