style.css

#entry1 {
    background-image: -gtk-gradient (linear,
                                     0 0, 1 0,
                                     color-stop(0, #f00),
                                     color-stop(1, #0f0));
    color: blue;
    font-weight: bold;
}

button {
    /* If we don't put it, the yellow background won't be visible */
    background-image: none;
}
#label1:hover {
    transition: 500ms;
    color: red;
    background-color: yellow;
}

combobox button.combo box {
    padding: 5px;
}
combobox box arrow {
    -gtk-icon-source: none;
    border-left: 5px solid transparent;
    border-right: 5px solid transparent;
    border-top: 5px solid black;
}