Struggling with the button

Hi all,

I am struggling to customise the Buy Now button. As I understand from other posts, there is not much we can do to customise it? It would be great if I could change the colour and size etc. Is there any way to do it?
It is suggested that we use a photo and create a URL and add it to the button link, however, when I do that, it did not change anything.
Any idea?
There was already a custom created buy now button on my page. Is it possible to create a link with the button shortcode etc?