Hello,

Please try adding the following Custom CSS in the Global Custom CSS area under THeme Settings >> Custom CSS.

.swatch-on-single.swatch-with-bg.swatch-size-large {
    border-radius: 0;
}

input.minus {
    border-right: 0 !important;
}

input.plus {
    border-left: 0 !important;
}

Best Regards