I have been in love with the Solarized colour scheme(s) since I first laid eyes on them. Before I found Solarized, I would always opt for harsh, high-contrast colour schemes consisting solely of bright colours on black backgrounds. What I like about Solarized is that it claims to be non-arbitrary. Most colour schemes have the feel that everything was picked because the author just liked the look of it. Solarized appears to have some method, though not one which I can understand. So, in summary, SCIENCE!

(use-package solarized-theme
  :ensure t
  :pin melpa-stable
  (setq solarized-distinct-fringe-background t
        solarized-use-variable-pitch nil
        solarized-high-contrast-mode-line t
        x-underline-at-descent-line t)
  (load-theme 'solarized-light)
  (load-theme 'solarized-dark))

First, an explanation of the reason I load the light and then the dark version of Solarized. I do this so that both themes are 'approved' by Emacs, meaning that in ~/.emacs.p/custom.el – where I store my Customization settings, see Day 2 – the hashes for the themes are stored as follows:

    ("<hash-for-solarized-light>" "<hash-for-solarized-dark>" default))))

This means that whenever I choose to switch between the two versions of the theme, due to the lighting in the environment or the content of a buffer, I needn't worry about warning dialogs. The rest of the settings ensure that Solarized doesn't visually affect other aspects of the Emacs instance.