I just saw a black background today plus (free) theme yesterday. I'll find it and include a link. It would have either been on blueroomsolutions.com or themes4bb.com.
>> Edit... OK the black background Today Plus theme I saw yesterday is @ themes4bb... It's @
Bold Today Plus for 4.2 - Themes4BB.com. It's not a 4.5 theme but, when I checked it out, it did work on my 8310 w/OS 4.5.0.55. There are also several additional good Bold backgrounds in that same thread.
There is a black background wallpaper somewhere. As long as the theme allows for a change to the background on the home page, you can do it that way.
>> Edit... OK - there's one here on page 19 of this thread of 320 x 240 (curve) wallpapers...
320x240 wallpaper - BlueRoomSolution