Restyling the Keep Unread Button, makes it  more 'visible'

I discussed this wtih @Raydiation on IRC. Keep Unread Button deserved some treatment as it was hardly visible. Both of us are not sure whether to display Keep Unread always or not. Currently it is always visible, but won't mind adding a display:none; to make it visible the way it was before. cc @jancborchardt :)