Docs issue: diagrams not readable in dark theme

[Spoonbender]

The diagrams found here are not readable when viewing GitHub using the dark theme.

Steps to reproduce:

1. Set your GitHub appearance preferences to use dark mode
2. Browse to the wasmer examples readme.md at https://github.com/wasmerio/wasmer/blob/master/examples/README.md
3. Attempt to read the text inside the diagram blocks (see screenshot below)

Expected behavior:
Reading succeeds and the information is conveyed.

Actual behavior:
Reading fails and the Wasmer main workflows remain a mystery.

Possible workarounds:

1. Configure light theme on your GitHub appearance preferences, or
2. Browse the docs in Incognito mode

[syrusakbary]

Thanks for the ping! The new diagrams have white backgrounds (instead of transparent ones) so it's easier to read in Github with the dark theme!