switch_themes cap belongs to Adminstrator role by default. Simplest is to promote an editor to administrator if you want him/her to switch_themes.
There is no built in way to edit capabilities for a specific user, only the role they have been assigned in each site can be changed.
To add capabilites to a role(so all users of the same role will enjoy the new cap) can be done with a plugin as simple as this (activate network wide or site by site).
<?php
/*
Plugin Name: Editor Role Gets Switch Themes Cap
Plugin URI:
Description: Editor Role Gets Switch Themes Cap
Author:
Author URI:
*/
$edit_contributor = get_role('editor');
$edit_contributor->add_cap('switch_themes');
?>
