Thanks for posting your com.apple.Boot.plist and how many options have you added?
Let's rewind for a minute and just start off with a simple one (I have populated it with relevant data from your file), and you can add booter options back in later if you need them.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Graphics Mode</key>
<string>1024x768x32</string>
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string></string>
<key>Timeout</key>
<string>5</string>
<key>Default Partition</key>
<string>hd(2,2)</string>
<key>Theme</key>
<string>Blackosx_3D_Theme_640x480</string>
</dict>
</plist>
As for why you can't get the theme to load, I'm not sure as your folder structure looks okay.
You say that whatever you try, the default theme always appears?
Well the boot file has the default theme built in, so to test if the built in theme is being loaded and not the Default theme from the Themes folder then try this...
Backup your current device_hfsplus.png from the /Themes/Default folder and replace it with the attached one from my Christmas theme (unzip it first of course). Then when you boot, see what the device icon for HFS+ looks like. If it's the default version then you know the built in theme is being used.