ReFrame logo in README does not show up correctly in Github's dark mode

[vkarak]

Screenshot:

Thanks @branfosj for spotting this.

[teojgo]

There is a related issue here: https://github.com/github/markup/issues/1416

[teojgo]

It is possible to distinguish logos based on the light/dark mode: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to