You would need to extend the theme with an update background color. See https://github.com/roundcube/roundcubemail/wiki/Skins for the information on extending the theme.
Plugin not correct work on modern version of Roundcube. I install it manually by downloading to ...roundcube/plugin/rc_login_background.Now I test it and plugin required modification in custom.php like this: ....... $img=date("n").".jpg"; echo " #layout-content { background-color: transparent !important; }body {..........and another one in rc_login_background.php you must replace custom.css to custom.php:function render_page($p) { $this->include_stylesheet('custom.php'); }so now it work for me