[README] Show the logo in an appropriate color scheme for the dark theme

Root Cause

• The color scheme of the logo is not suitable for GitHub dark theme

Notes

• Theme for GitHub markdown: https://github.blog/changelog/2022-05-19-specify-theme-context-for-images-in-markdown-beta/
  • Is this applicable to the NPM page as well?

Possible Workaround

• Use a bit lighter grey color for the texts