Lighbox customization

Hi,
I have just bought Colibri PRO theme which is working great. When I click on button I can choose on the link to be redirected to, Same Page Different Page or Light Box. If I choose the lightbox I can chose between Auto, iFrame, Image and Video. Now I would like to use Link -> Light Box -> iFrame and use this to create my customize frontend login. When I do everything is fine but the light box is as big as the parent window. I’d like to be able to customize the Lightbox. I didn’t installed any plug in, so I believe the Lightbox was either coming from WordPress or Colibri PRO.

I understand I can modify and customize elemnts using CSS (styles.css or additinal css and so on) BUT where is located the code for the lightbox? Can anyone please point me in the right direction? Can you please help just to modify any parameter for the Lightbox.

Sorry maybe it’s a simple and stupid question, but I’d appreciate the help.

Thank you