You can use built-in package manager (package.el) or do everything by your hands.
Link to MELPA
(require 'package)
(add-to-list 'package-archives
'("melpa" . "http://melpa.milkbox.net/packages/") t)
I personally prefer MELPA, 'cause it hourly updates its packages from upstream source.
Then use M-x package-install RET flux-light-theme RET to install
the theme. Use M-x customize-themes to change your current theme.
Download the theme to your local directory. You can do it through git clone command:
git clone git://github.com/convict-real/Flux-Light-Theme.git
Then add path to flux-light-theme to custom-theme-load-path list — add the following to your emacs config file somewhere (.emacs, init.el, whatever):
(add-to-list 'custom-theme-load-path
"/path/to/flux-light-theme/")
Use M-x customize-themes to change your current theme.