Hi,
1. That's because the "force image size" option of the configuration is turned on.
Deactivate it and you won't get the white strips on images that don't fit the dimensions you entered for them.
The strips color cannot be changed as there is no option to change their color.
2. The image of the popup is the original image so it's not possible to make it bigger (it would just be blurry).
Instead you should upload again the image with a greater resolution.
If your original image is bigger than it is displayed in the popup, it's because you set the "image width/height" option of the configuration which resize the original images on upload. Empty these fields before uploading again the image and then it will display with the same size as the one you uploaded.