So you have two buttons.
First one is for Discount Coupons:
<button type="button" title="Apply" class="button apply-coupon" value="Apply"><span><span>Apply</span></span></button>
It has class .apply-coupon and .button so you can style it however you want in your CSS files by making:
.button.apply-coupon {
/*Your styles go here*/
}
The background that you see under the text is caused by line 167 in opc.css
.opc-wrapper-opc .discount-block .button span span {
background: #00909E;
color: #FFF;
font-size: 16px;
font-weight: bold;
text-transform: uppercase;
text-shadow: 0 1px 1px #02484F;
padding: 0 20px;
}
The span that's inside the button is styling text wrapped in the button with green background. Delete the span inside the button or add additional css like:
.opc-wrapper-opc .discount-block .button.apply-coupon span span{
background:none !important;
}
The green background on:
<button type="button" title="Place Order" class="button btn-checkout opc-btn-checkout"><span><span>Place Order</span></span></button>
Is caused by the same style in opc.css but on line 216.
.opc-wrapper-opc .btn-checkout span span {
background: #00909E;
color: #FFF;
font-size: 18px;
font-weight: bold;
text-transform: uppercase;
text-shadow: 0 1px 1px #02484F;
You can change it by deleting that background-color for button or adding css:
.opc-wrapper-opc .btn-checkout span span {
background: none !important;
}
P.S. Check your whole OPC.css file as I see more duplicated styles for that span wrapper in buttons (i.e. search your file for: background: #00909E;)
Edit:
Now on SPAN element:hover you're changing your background color to #f55 just like on button. But on button you added
transition {
color 300ms ease-in-out, background-color 300ms ease-in-out;
-moz-transition: color 300ms ease-in-out, background-color 300ms ease-in-out;
-webkit-transition: color 300ms ease-in-out, background-color 300ms ease-in-out;
-o-transition: color 300ms ease-in-out, background-color 300ms ease-in-out;
}
So you have to add same transitoin on span hovering as span is immediately changing color to #F55, while button is animating transition effect which occurs with 300ms later color change.