If you are planning add VCL Styles support to a large set of projects you have to build a custom tool or script, Try these options.
Option 1, embedding the VCL Style as a resource in the exe: Using this option you need to modify at least 2 files per project , the .dpr file which set the current VCL Style and the .dproj file which need a reference to the VCL Style file to embed the Style as a resource.
Option 2, using the VCL Style from a external file, Using this option you need to modify the .dpr file which set the current VCL Style adding the necessary code to load the style from an external file.
To modify the .dproj file you can use an automation tool or scripting language which supports XML.
For modify the .dpr file you need to build a custom Application ideally using a Delphi Parser like DelphiAST or Castalia-Delphi-Parser adding the necessary code to set the current VCL Style.