In Emacs using M-x load-theme
will load a color theme, however it
will also leave any existing color themes loaded too.
This is a bit of a problem, because Emacs will slow down when a few color themes are loaded.
Select themes is designed to work around this by disabling any currently enabled themes before loading the selected theme.
It's also not possible to explicitly set the Emacs default theme using load-theme, so Select themes takes care of that too, allowing you to simply select default.
M-x select-themes
Ido or whatever function you have set to override completing-read
will be used to manage selection.
Note: This package is a simplified, UI agnostic version of helm-themes.