You need to add transition to your button: something like this:

.btn {
    float: left;
    font-size: 18px;
    font-weight: 500;
    padding: 13px 39px;
    margin-bottom: 0;
    background-color: transparent;
    text-decoration: none;
    text-align: center;
    cursor: pointer;
    border: 2px solid #2F3C7E;
    border-radius: 7px;
    color: #2F3C7E;

    // Transition added (You can add transition to a specific thing like color or background color or just use all)
    transition: all ease-in-out 0.2s;
}

also have a look at this: https://developer.mozilla.org/en-US/docs/Web/CSS/transition