Sorry, in my first question I hadn't realized what I actually wanted! It was an answer to my question, but I hadn't actually asked the right thing. However I was able to figure out what I had to do from the response you gave me! So thank you so much!
For anyone in the future who wants to do the same thing, I ending up wanting to adjust the lightbox form button so that it matched the container size, like the attached image. The code I used that worked for me was this:
.lightbox-handle{
width: 100% !important;
height: 100% !important:
}
Since I only wanted this effect on this page, I put the code in the advanced section of the page's settings with html's style tags. If you wanted it everywhere you can put it where Lesum said to, without the style tags.