Adapt starlette's logo to the current theme

[AbduazizZiyodov]

Summary

Discussion: #2797

Made Starlette's logo is adaptive to the current user theme on the readme file.

Dark mode:

Light mode:

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

References

• 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

[Kludex]

Thanks so much for this! :)