In the first example you are serving the same file for all the urls:
/
/base.css
/react.js
Have a look in the developer tools in Chrome or use curl to the test the responses. You need to do some routing, so that / is handled by the template handler and the static files are handled by http.FileServer. It usually helps to put all the static files in a public sub folder.
If all this is a bit new to you, might be worth having a look at one of the web frameworks, such as gorilla/mux or revel:
Hope that helps,
Donovan.