metamask-extension/ui/app/components/app/signature-request/signature-request-footer/index.scss

.signature-request-footer {
    display: flex;
    border-top: 1px solid #d2d8dd;

    button {
        text-transform: uppercase;
        flex: 1;
        margin: 1rem 0.5rem;
        border-radius: 3px;
    }

    button:first-child() {
        margin-left: 1rem;
    }
    button:last-child() {
        margin-right: 1rem;
    }
}