This is a very basic example modeling a video inside a .video-container with a .video-thumbnail element and a .play-button element. All you have to do is choosing a background image for the play button that will be rendered with a transparent background as long as the transparency is handled in the picture. Otherwise you should opt for vector-graphics solutions like I found here https://css-tricks.com/making-pure-css-playpause-button/ and here https://codepen.io/chrisdwheatley/pen/WNZjjJ for example.
This is my attempt:
<style>
.video-container{
position: relative;
/*faking its size because I don't have a real video*/
width: 320px;
height: 180px;
background: blue;
}
.play-button {
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
background: transparent;
/*here you can choose your picture for the play button*/
background-image: url('http://...');
}
</style>
<div class="video-container">
<img class="video-thumbnail" />
<div class="play-button"></div>
</div>