The Tooltips style can be changed by setting CssClass property with CSS like following:

    .green-white-tooltip {
        background-color: green;
        color: white;
    }

    .red-yellow-tooltip {
        background-color: red;
        color: yellow;
    }

    .gradient-tooltip {
        background-color: greenyellow;
        background-image: radial-gradient(circle at top right, yellow, #f06d06 50%);
        color: black;
    }

    .text-style-tooltip {
        background-color: lightyellow;
        color: black;
        font-style: oblique;
        text-shadow: 2px 2px 5px green;
    }

    .strong-round-tooltip {
        background-color: palegreen;
        color: black;
        border-radius: 20%;
    }