datacarpentry
Explicitly set dark mode images to avoid varnish image color inversion
As explained in #349, varnish defaults to inverting all images included in the curriculum for dark mode. This is a problem as most of the material included here relies on the raw 'light-mode' images appearance.
This PR closes #349 by explicitly creating copies of all images that need to be preserved as-is in light-mode by appending -dark to the image file names (discussed here).
Originally I was making a copy of all images, but I found several histograms and/or images that had a transparent background. This caused the black text on black background (in dark-mode) to be unreadable, so I either updated the images to have a complete white background, or left varnish doing its magic to invert the color scheme. The only side effect of this choice is that there's no consistent aesthetics to all graphs, but there were inconsistencies between fonts and style before anyway.
I tried to go through all the content to see if all images and text references still make sense with these figures, but it would be great if you could double-check if I missed something.
