Glad you sorted it out!
Just to clarify what was going on there, the Add to Cart code for items using Variations/Variants is actually an HTML form, where the Add to Cart button itself is actually an <input type="image"> that submits the form, rather than a linked <img> tag, so CSS rules affecting <input> elements would affect that type of Add to Cart button. Since our standard button code already specifies that button with a class="ec_ejc_thkbx" attribute, your CSS fix could just do something like this:
input.ec_ejc_thkbx {width:auto; background:none; border:0;}