Colour of selected radio button seems backwards in Twenty Seventeen + Woocommerce

What do you mean on a dark background? Did you change the background color? My default twenty seventeen & woo install has a white background.

Anyway, you can make CSS adjustments by going into the admin > Appearance > Customize > Additional CSS.

You can inspect the CSS on the page by hitting f12. I would recommend inspecting the elements and adjusting the CSS colors values to what you like, then copy the changes into the admin customizer.

The below screenshot shows the elements you want to adjust. Use the inspector and click on the ::before inside the active label.

After you have the colors you want copy the changes into the customizer. For example, pasting the following CSS will switch the colors of the radios (black with white & white with black).

.wc_payment_method input.input-radio[name=payment_method]:checked+label:before {
    background: #fff;
}

.wc_payment_method input.input-radio[name=payment_method]+label:before {
    border: 2px solid #000;
    box-shadow: 0 0 0 2px #fff;
    background: #000;
}