Two install paths — same theme either way.
From JSR (recommended)
dune theme:install jsr:@dune/theme-gale@1.0.0 --activate
From a ZIP
Grab the release ZIP from dune-themes releases
and extract it into themes/gale/, then set:
# site.yaml
theme:
name: gale
JSR is easier to keep updated; the ZIP is better if you want the theme vendored in your own repo.