BuddyPress.org

Sorry, i’m wrong. I checked again and effectively, the alternate colors are weird.
Add this to theme’s custom option (wp-admin/themes.php?page=WeaverX > tab main option > tab fonts&custom > custom css rules)

It’s a rasta example, so you can see how it works. You just have to modify the colors to your needs.

div.message-content {
color: yellow!important; /*#FFFFFF;*/
}

#buddypress div.message-box {
background-color:green!important; /* transparent!important; */
}

#buddypress div#message-thread div.alt {
	background: red!important; /* #CCC; */
}

May this help. 😉