Is it possible to apply any themes to improve the appearance of Evolution?

Evolution uses the same theme as your other desktop applications but it will probably look as dull as it is now. So you are out of luck there.

To make this worthy answering I can advise on some cool themes made for Ubuntu 11.04 that also adjust evolution. This will install 3 themes:

sudo add-apt-repository ppa:tiheum/equinox
sudo apt-get update
sudo apt-get install gtk2-engines-equinox equinox-theme
sudo apt-get install faenza-icon-theme

named Equinox Evolution Dawn, Equinox Evolution Dusk and Equinox Evolution Midnight and a Faenza icon set that go with it.

Default Ambiance:

Midnight:

Dusk:

(dawn is another light version of this)

But basically it is the same layout with the same icon set and another color.