Na jednoduché prechody ti stačí nastaviť transition (transition-property, transition-duration, transition-timing-function), napr. (netestované):
.i_order {
background: #fff;
color: #1b75bb;
transition: all .2s ease-in;
}
.i_order:hover {
background: #1b75bb;
color: #fff;
}