Go to your cookie configuration and click on the gear icon in the top-right menu, then click on "Colors and Styleguide".
You will be redirected to the customization screen. You will have to check the first checkbox on top of this page : "Use a custom button"
You will then have access to new customization features? You will just have to click on the first button just under the checkbox you just checked, named : "Pick an image".
Here you have two options !
Firstly, you can go for a personal image that you created. You will have to import it by dragging and dropping it in the box, or by clicking on the box to open your file explorer.
If you don't have any personal pictures, but you still want to change your widget button, you can do so by picking one of the suggested images.
Finally, after being through those quick steps, you can now manage every detail of your new button (font size, height and width of the button,..)