How can I customize the lightbox?

To customize the lightbox, you need CSS knowledge, otherwise you won't be able to achieve what you are looking for. You should inspect the codes of the lightbox, write custom CSS codes which you could put to your theme's CSS file or to the slider's HTML control.

If the codes doesn't seem to have affect, check out this documentation: Why isn't my CSS code working?

You can also find a documentation on Lightbox CSS codes people commonly want to modify which might be useful for you.