Surgeweb has a layered customisation interface that allows basic branding customisation of the interface and detailed extension / customisation of the interface all without modifying the underlying template based interface.
This makes your life MUCH easier during upgrades as you do not have to redeploy your template changes each time you upgrade surgemail (as you had to with old webmail).
This product is no longer recommended, but here are some suggestions on its customisation.
SurgeMail web interface is based on the same flexible template technology. However there is normally less need for this.