Allow New Users to Personalize Overall Color of Default Theme During Installation

Consider giving new users the option to change the color of the default desktop theme during installation. For example, on step 5 of the installation, the installer could display "Optional: personalize the color of your desktop," followed by a single color-picker button, which, upon being clicked, displays only a handful of sensible predefined color options. Rationale: (1) no single color scheme will ever be universally liked; (2) everyone enjoys personalization; (3) how to personalize a theme's colors is currently non-obvious for many users -- it requires that they click their way through System -> Preferences -> Appearances -> Theme -> Customize -> Colors.