To do that theme customizer needs to be able to edit batches of images on the fly. Change saturation/hue/etc and ... actually, that's conceivably doable - just future themes that support this would have to come with all-white icons or something, so that such transformations could be done on the base, then saved somewhere...

White icons aren't actually necessary, but I think they'd make it more practically useful, since you can modify RGB or saturation/hue and transparency values from there. (Unless you're talking about the scroll 'bar' that you see in conversations and microb, that don't actually allow scrolling, but scroll as you move the screen around.)

Although personally I have always been a fan of the Marina theme, and no other theme has ever appealed to me as much, and have just been modifying it slightly for months when I get the time. (I like it, but purely desaturated colors are even more appealing to me, so I've been going for white versions of all the blue icons....)