Shapes in readme images can't be seen with GH's dark mode

[jjant]

Basically the title, shapes are black with a transparent background, which makes them invisible in dark mode:

[ianmackenzie]

I wonder what the best solution is here...it looks like it's now possible to specify theme-specific images but that will almost certainly mess up how the README is displayed on package.elm-lang.org (both images will get displayed). I guess I could do something like use theme-specific images on master, and for every published version of elm-geometry create a release branch where the README only has the light mode images so it renders properly on the package site...a bit annoying but that's the only approach I can think of that makes things look good in all three cases (GitHub light mode, GitHub dark mode, Elm package site). Thoughts?

[jjant]

@ianmackenzie I think the common solution for these kinds of things is to use outlines and fills for everything (as they do for subtitles, for ex).

If you do that, and/or use exotic colours (magenta, bright greens), you should cover most cases.