StyleClass does not work with apex:commandButton Salesforce ?

Below is the code for CSS to change button styles

<apex:page >
        <style type="text/css">
                    .myClass {
                             color:white;
                             background-color:#00CC00;
                     }
         </style>
          <apex:commandButton styleClass="myClass" value="Save" action="{!someAction}">
</apex:page>

If above CSS is not reflecting to your button then change the CSS by adding Important to CSS
like below

<style type="text/css">

        .myClass{

                    color:white !important;

                    background:#00CC00 !important;

         }


</style>