So task is simple, but not for me - green Gnome Extensions Development noob. Of course I googled this, but didn't find something that can be helpful here.
extension.js:
https://pastebin.com/TqDVp8Yz - because 'your post mostly code'.
stylesheet.css:
.poppy-button {
background-size: 20px;
width: 20px;
height: 20px;
background-image: url("Resources/poppy.svg");
}
.poppy-button:active,
.poppy-button:focus,
.poppy-button:checked,
.poppy-button:hover {
background-image: url("Resources/poppy-active.svg");
}