Program "Useful File Utilities" have internal "Replacer" utility which allow you to solve your task. Internal "Replacer" utility have such functions: Replace with, Insert before, Insert after, Insert between, Delete, Insert to head file, Insert to foot file. You can add CSS support to your website with using function "Isert after". Just enter <head> in the first text field. In the second text field enter
html text which you want to add after <head>. Also you need to select files which you want to change with using mouse or keyboard. Then press to "Start Replace" button and operation will start. This utility allow you to work with all html files. Also you can find other useful function in program "Useful File Utilities". E.g. using tility "Finder" you able to find any documents in your computer with
boolean request like in popular internet searches.
You can download this program freely from website: