How can I change the homepage slider play icon?

Instead of the circle with a triangle in it, I’d like a button.

How can I change the play icon on the homepage slider; the one that activates the video lightbox?

@webmagnets Do you need this modification in the Inspiro theme?

This is not a simple modification, but you can try to use the following CSS code and adapt it per your needs:

.portfolio-popup-video, .slides .popup-video {
    border-radius: 0;
    border: 2px solid rgba(255,255,255,.8);
    display: inline-block;
    width: auto;
    height: auto;
    margin: 40px 0 0;
    line-height: 2;
    position: relative;
    outline: none;
    transition: none;
    opacity: 1;
    animation: none;
    padding: 3px 20px;

.portfolio-popup-video:after, .slides .popup-video:after {
    display: inline-block;
    margin: 0;
    content: "Your Text";
    vertical-align: middle;
    -webkit-font-smoothing: antialiased;
    color: #fff;
    font-family: inherit;
    font-size: 20px;
    line-height: inherit;
    font-weight: 600;

You can see that the text of button is included in this line:

content: "Your Text";

Awesome! Thank you. That worked.

Is there anything I should be testing to make sure it doesn’t break the lightbox on some certain browser? Is there a possibility of that?

No, you shouldn’t worry about that.