To give you an idea of the possibilities, this is the theme that I'm using right now (LoginToLion):
Yes, that's what I see every time I boot my Hackintosh. Not bad, right? Follow these steps to install this bootloader theme yourself:
1. Download a desired theme. Unfortunately, there's not just one big repository for all bootloader themes, but the tonymacx86 Customization forum has a few, as does the Theme Park section of the official forums for Chameleon bootloader (which Chimera is based off).
2. Unzip the theme (almost all themes are downloaded as a zip file). Open theme.plist in the theme folder with TextEdit, and change the parameters under "screen_width" and "screen_height" to whatever your monitor resolution is.
3. Go to /Extra and open the plist file org.Chameleon.boot.plist. Between <dict> and </dict> in the file, add the following lines.
Replace 1920 and 1080 with the width and height of your monitor, respectively. Then save the file. In Mac OS X Snow Leopard, you won't be able to save the file normally because it's a locked system file (in Lion, you can unlock system files). Instead, save a new copy of org.Chameleon.boot.plist to somewhere random (make sure that you save it as a .plist file). Then delete the old copy of the file and replace it with the new one.
Editing theme.plist and org.Chameleon.boot.plist will make Chimera attempt to run at your given resolution. However, Chimera can only display resolutions that are programmed into your graphics card (these are called VESA resolutions). For example, my graphics card can only display the following VESA resolutions at the Chimera bootscreen: 1024x768, 1440x900, and 1680x1050. Nevertheless, you should still edit theme.plist to your desired resolution. Chimera should display whichever one of these resolutions is closest to the width and height that you entered. If you have a widescreen monitor but your graphics card doesn't have any widescreen VESA resolutions, check out my workaround method.
4. Copy the theme folder into /Extra/Themes in your main hard drive. In /Extra/Themes, you'll also see the folder for the Default theme. If you want to be really thorough, just delete that folder and rename the new theme folder "Default". If you want to apply your theme with less of a brute force method, check out Step 3.
5. Go to /Extra and open the file org.Chameleon.boot.plist. Add the following line between <dict> and </dict> in the file:
Obviously, replace "theme name" with the name of the folder that contains the new theme in /Extra/Themes.
Reboot, and enjoy the new look.