Create the css class you need 

e.g a class for the button hover border: 

.mybutton-hover-red:hover{
border:1px solid red!important;
}

and add this to the custom override file at:

content/skins/tempo/custom_overrides.html


After this just add the

mybutton-hover-red as a class suffix for the buttons where you want to use that hover style:

http://your domain/admin.php?p=design_images&mode=button&pg=3