Clone this repo in a custom folder at the root of your gitea install.
Edit app.ini to set the following in the [ui] section:
- DEFAULT_THEME = bthree-dark
- THEMES = bthree-dark
Run the gitea binary with the following arguments (or do the clone into Gitea's own custom directory, merging with your conf dir).
- --custom-path pointing to the repo directory
- --config pointing to the custom app.ini
Update your user preferences to set the new theme, or it will keep showing the old one.

Alternative setup with Docker

Initialize docker-compose.yml following Gitea's basic instructions.
Install Gitea following Gitea's installation instructions.
Edit app.ini in [Gitea's root]/conf to append the following:

[ui]
DEFAULT_THEME = bthree-dark
THEMES = bthree-dark

(Docker Gitea's root directory defaults to /gitea/gitea/ from installation root.)

Clone this repository to Gitea's root directory, so that its contents are merged. (Gitea's custom directory defaults to Gitea's root.)

Gitea's root flat directory tree should look similar to the following, if done correctly:

.
├── actions_artifacts
├── actions_log
├── attachments
├── avatars
├── conf
├── gitea.db
├── home
├── indexers
├── jwt
├── log
├── packages
├── public
├── queues
├── README.md
├── repo-archive
├── repo-avatars
├── sessions
├── sphinx
├── templates
└── tmp