Tweak/readme logo

[rdwebdesign]

Please make sure you

1. Base your code and PRs against the repositories developmental branch.
2. Sign Off all commits as we enforce the DCO for all contributions

What does this PR aim to accomplish?:

Add code to choose the right logo for dark themes.

How does this PR accomplish the above?:

Following the instructions from here.

What documentation changes (if any) are needed to support this PR?:

none

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

• I have read the above and my PR is ready for review. Check this box to confirm

[yubiuser]

We also need to change this in the core repo. (pi-hole/pi-hole#4735)
It still uses #gh-dark-mode-only which is considered deprecated

The old method of specifying images based on the theme, by using a fragment appended to the URL (#gh-dark-mode-only or #gh-light-mode-only), is deprecated and will be removed in favor of the new method described above.