Change “next” button

I would like to change colours for 3 buttons:
– Place the order
– Next (during checkout)
– Pay and buy at the end of checkout

I have an issue with “Next” button. My change in Custom CSS causes that this button’s color is set randomly. Once as main theme colour, once my selected one. There is my code. I would appreciate any hint.

.button#place_order.button {
    background-color: #42b72a;
}

.button#place_order.button:hover {
    background-color: #2f821e;
}

.button.next {
    background-color: #42b72a;
}

.button.next:hover {
    background-color: #2f821e;
}

.checkout-button.button {
    background-color: #42b72a;
}

.checkout-button.button:hover {
    background-color: #2f821e;
}

The page I need help with: [log in to see the link]