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The problem is that your images are only 300px, and so on Xtreme by default they max out at that width so the quality stays correct.
On the other theme for some reason, there is a CSS rule that forces the Width to a larger value, resulting in the images being scaled up, and therefore slightly blurry.
We could give you a Custom CSS rule that would do the same, but the best would be for you to select larger resolution version of the images.