Can you really grant Editors a chance to manage theme icons? Automatically, only Managers can perform so.
In WordPress 3.2.1 the capacity is
switch_themes is going to do simply permit you to activate another theme in the available ones.
I am afraid you are gonna need to make use of a wordpress plugin to do this, since altering Icons is really a switch_themes capability unavailable to Editors. You'll need a wordpress plugin that changes roles and abilities (like Capability Manager). Check a similiar topic about this issue in Wordpress Forums.