Added white background to all images (#630)

[bolshoytoster]

The images don’t work well in dark mode (#630), I added a white background to all .png files with imageMagick:

find -name '*.png' -exec convert {} -background white -alpha remove -alpha off {} \;

[harsh-batheja]

@donnemartin Please merge the pr.

[bolshoytoster]

@harsh-batheja I've changed the backgrounds to grey, as you suggested here. The images that already had a background have stayed white though.

[benjo456]

Can this be merged please? Having real difficulty reading the images with dark mode. Thanks

[bolshoytoster]

@benjo456 you could use my fork instead for now.