XF 1.2 Set background colour, and conditionally change the background image



I am using the style Scroll by XenCrea. It gives me the option to set a background image, which is very nice. However, my issue is that some people have very high resolution monitors. As it currently is, for those people when the background image is a lower resolution than their monitor, the rest of the background is white - and that doesn't look very good.
I would like to change the background colour which is not filled by the background image to black (because my background images are quite dark), but I would also be very interested in knowing if there is some way to detect the resolution of the member and then conditionally change the background to a higher resolution image if he has a higher resolution monitor than that of the default background image (which has a small filesize and in my opinion looks better) so that they don't have to see any non-image background.
I'm not very experienced with template editing, so please bear with me if I am asking stupid questions :oops:... Any assistance is much appreciated, though!