Add Deep Dark theme (#138)

Hi! The Deep Dark theme link added.

Related issue: https://gitea.com/gitea/awesome-gitea/issues/137

Co-authored-by: Yuri Plashenkov <yuri@plashenkov.com>
Reviewed-on: https://gitea.com/gitea/awesome-gitea/pulls/138
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: plashenkov <plashenkov@noreply.gitea.com>
Co-committed-by: plashenkov <plashenkov@noreply.gitea.com>

README.md

@@ -199,6 +199,7 @@ A curated list of awesome projects related to Gitea and its soft-fork instances.
 - [Dark Arc](https://github.com/Jieiku/theme-dark-arc-gitea) - Dark theme with high contrast, based on arc-green.
 - [Dark Blue](https://gitea.artixlinux.org/artix/gitea-dark-blue) - The dark blue Gitea theme used on [https://gitea.artixlinux.org](https://gitea.artixlinux.org)
 - [Dark Red](https://github.com/iamdoubz/Gitea-Dark-Red-Theme) - Dark red theme by iamdoubz
+- [Deep Dark](https://github.com/plashenkov/gitea-theme-deep-dark) - A deep dark theme based on arc-green and featuring unified headers.
 - [GitHub](https://github.com/Rainnny7/gitea-github-theme) - A theme to make Gitea look and feel like GitHub.
 - [Matrix](https://github.com/TylerByte666/gitea-matrix-template) - Neon-green with a matrix-inspired background
 - [Pitch Black](https://github.com/iamdoubz/Gitea-Pitch-Black) - Pitch black 1.14+ theme used on [https://git.dou.bet/iamdoubz/Gitea-Pitch-Black](https://git.dou.bet/iamdoubz/Gitea-Pitch-Black)