live-build uses syslinux and some of its derivatives (depending on the image type) as bootloaders by default. They can be easily customized to suit your needs.
In order to use a full theme, copy /usr/share/live/build/bootloaders into config/bootloaders and edit the files in there. If you do not want to bother modifying all supported bootloader configurations, only providing a local customized copy of one of the bootloaders, e.g. isolinux in config/bootloaders/isolinux is enough too, depending on your use case.
There are many possibilities when it comes to making changes. For instance, syslinux derivatives are configured by default with a timeout of 0 (zero) which means that they will pause indefinitely at their splash screen until you press a key.
To modify the boot timeout of a default iso-hybrid image just edit a default isolinux.cfg file specifying the timeout in units of 1/10 seconds. A modified isolinux.cfg to boot after five seconds would be similar to this:
If you want to use a personalized background image that will be displayed together with the boot menu, add a splash.png picture of 640x480 pixels.
When creating an ISO9660 binary image, you can use the following options to add various textual metadata for your image. This can help you easily identify the version or configuration of an image without booting it.