Thank you for your reports @Globber.
This looks like an additional amount of padding is being adding to the :onhover. Should be a simple enough fix and I will post up an update on this later today.
This is actually intentional by design. We could perhaps lighten the button a touch more.
They are pretty much the exact same skin. Quickbox-dark however is there for anyone whom would like to experiment with making changes to the skin without breaking their Original 'Club-Swizards` skin. So, this theme is a courtesy and a playground of sorts.