The lightbox is using a background in a div for the button. So you can create a CSS custom code to override its styling and do it the way you want. Here is a code that I enriched for you so you get control over the main visual characterstics of the elements.
In order to see the results of this code in the designer, put this code inside of an Embed component inside of your page, anywhere. (put it under the body for conveniency).
The color red property is for you to well visualize the div when you’ll style it, remove it when done.