Yes, that's down to the colour gradient: in X-Mas, the colours run dark to light suggestive of the dip you refer to, in the other image, the colours run light to dark, suggestive of the bulging you refer to.
(It would have been easier had you just provided a before and after screenshot to start with...)
Unfortunately changing it is not particularly easy, because the same gradient is used in a bunch of places, which means it's going to be a change to the template file, a change to the CSS file and creating a new image...