karaf/src/main/resources/org/apache/karaf/webconsole/karaf/internal/feature/features.css

div.actions ul li a {
    width: 16px;
    height: 16px;
    display: block;
}

div.actions ul li a.installed {
    background: url("images/feature_uninstall.png");
}

div.actions ul li a.uninstalled {
    background: url("images/feature_install.png");
}