Update README to include GNOME/GTK version of Dark Mode macOS alias. PR:#2963

[skttlock]

Hello!
Thanks for making a great tool & sharing it with the world.
I've just converted the script provided under the "Dark Mode" header from the given macOS script to a Bash script for GNOME/GTK Desktop Environments. I'll send a PR to include this in the same section.

My personal setting uses the gruvbox theme options, but I'll change it to match the given script.
This was developed on Fedora 39 GNOME.

Please let me know if I can improve the alias/script at all! Noob in Bash scripting, always looking to further my skill!

PR submitted: #2963
p.s. reading the contribution guidelines I think I'm not supposed to submit an entry in the changelog. Let me know if I'm wrong about this.

Thanks again!

[skttlock]

Editing to add that I am trying to figure out how to properly label this issue and the related pull request. Sorry!