Download free OxyBird Discreet Oxygen Theme for Thunderbird
This is a theme based on the oxygen icon set. It is named OxyBird because it supports also Lightning and SunBird.
The theme is made by Wichien Maat. Except for using Oxygen icons, the theme has a pleasant side effect: it won’t interfere with your main Windows theme.
* You may also like to check how to customize almost everything in the interface of Thunderbird