How to Change Hover Over Colour on Submit Review Button

Try replacing your CSS with this:

.glsr-review-form button.glsr-button {
    background-color: var(--accent);
    border-radius: 3px;
    font-family: Mouse Memoirs;
    font-size: 26px;
    justify-content: center;
    padding: 15px 20px;
    text-transform: capitalize;
    transition: background-color .2s ease-in-out 0;
    width: 100%;
}
.glsr-review-form button.glsr-button:hover {
    background-color: var(--base-3);
}