I search and searched but could not figure it out.
I could not figure out (1) the CSS code or (2) where to put it.
A simple white button with black font would work.
I did find the following in the documentations but do not understand it:
"Change the style of the subscribe/unsubscribe buttons
Both buttons are included in a td class="acysubbuttons" and with a class="button" on it so you can apply some styles via CSS using: td.acysubbuttons .button{...}
You can also add a style on td.acysubbuttons .button:hover to create an effect when the cursor is on the button."