Could you elaborate what do we mean by Green Marketo here? We do have social sharing buttons, but those are using the colors as used by the social platforms. I hope by social buttons, you mean the following - ?
If you just want to change the green button, then, from the button style, choose "Upload Image" option that'll replace the green button with your uploaded image for the button. But, this will not affect the Twitter, LinkedIn or Facebook's icons - they'll remain the same. You can upload any dimension for the button image.
If you want to change the icons also, then for every icon, you'll have to create a separate social share. Here's an example which you can import and use, or create your custom social share in the same way.