You know that grey Apple is exactly how a "real" Mac looks? Just saying.
But if you want to change that up, there are plenty of bootloader themes out there on the web. Just google it.....
The booloader theme is located in /Extra/Themes.
You would need to add your new theme to that /Themes directory.......and edit your com.apple.Boot.plist file with the following.
is the name of the theme file you downloaded and placed in the /Themes directory**
Thats the skinny on it.
PS: the voodooprojects website has a themes section.