[Layout and Design] Adapt the `README.md` banner to GitHub's dark theme

[Korb]

Summary

Currently, the banner is barely visible when GitHub's dark theme is enabled.

Steps to Reproduce

1. Open https://github.com/r-lyeh/Spectral#:Rr9ab:

Expected Behavior

Actual Behavior

(src/res/logo/bar1.png)

Environment

• OS: Microsoft Windows 10.0.26200.7462
• App/Browser version: Mozilla Firefox 147.0b4

Additional Context

src/res/logo/bar3.png and src/res/logo/bar4.png look equally good in both light and dark site themes.