Install the theme

Upload and activate

Go to Appearance → Themes → Add New → Upload Theme.
Choose the fern.<version>.zip file and press Install Now.
Press Activate.

Upload the zip exactly as it downloaded. Unzipping it first and uploading the folder gives you a theme with no stylesheet at the top level, which WordPress refuses to install.

After activating

Everything is in Appearance → Customize. The theme adds no admin menu of its own — its sections sit alongside WordPress’s in the Customizer:

Section	What is in it
Site Identity	Logo, title and tagline
Homepage Settings	The homepage title and the latest-posts list
Header	Layout, alignment and width
Blog	The listing, the date format and the archives page
Single post	What a post shows, and comments
Footer	The copyright line
Design	Type, color, links and width
Misc	Classic editor, classic widgets, search
Info	The version you are on, and where to go for help

Fern does not build pages or menus for you, and there is no demo import. Set a front page under Settings → Reading if you want one, then build a menu at Appearance → Menus and assign it to Primary menu — the header prints no menu until one is assigned.

Updating

Fern is not on wordpress.org, so updates are uploaded the same way the first install was. WordPress will not overwrite an active theme from the upload screen, so either:

deactivate Fern first by switching to another theme, upload the new zip, and switch back; or
delete the old theme folder over SFTP and upload the new one in its place.

Your settings survive either route. Customizer values are stored against the theme in the database, not in the theme folder, and your posts and pages are untouched.

Child themes

If you plan to edit templates rather than only add CSS, work in a child theme so an update cannot overwrite the edits. A child theme needs a folder with a style.css naming Template: fern and a functions.php that enqueues the parent stylesheet. For CSS alone you do not need one — use Customize → Additional CSS.