In `README`, screenshots with light backgrounds are very unpleasing to eye when using dark theme in GitHub

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer
- [X] I have searched other issues and found no duplicates
- [X] I want to request a feature or enhancement and not ask a question

### The problem

In the [README](https://github.com/AdguardTeam/AdGuardHome#readme), screenshots with light backgrounds are very unpleasing to the eye when using the dark theme in GitHub.

Screenshot:


### Proposed solution

Add screenshots of the app's dark theme when using dark theme on GitHub.

Useful link: https://github.blog/changelog/2021-11-24-specify-theme-context-for-images-in-markdown/

### Alternatives considered and additional information

_No response_