Quick Look not working for CSS and JavaScript files
You don't need any 3rd plugin plugins to Quick Look source code/ASCII text  files. Invoking Quick Look by pressing the spacebar on JavaScript/CSS source code file (which are plain text ASCII encoded) will show you the plain source code text.

Here's what QuickLook shows when invoking Quick Look on a JavaScript file named callback.js:



Installing QLColorCode using Homebrew by running brew cask install qlcolorcode and invoking Quick Look on the same file immediately thereafter shows this color coded output:



Follow the installation instructions on the project homepage to make sure your have correctly installed the plugin:


  This is a Quick Look plugin that renders source code with syntax highlighting, using the Highlight library.
  
  To install Highlight, download the library manualy, or use homebrew brew install highlight
  
  To install the plugin, just drag it to ~/Library/QuickLook. You may need to create that folder if it doesn't already exist.
  
  Alternative, if you use Homebrew-Cask, install with brew cask install qlcolorcode.