Why do the Roots theme CSS files not load (404)?

I took a risk and activated the theme to see what might happen. Roots needs to complete theme activation to add the rules into .htaccess. Until this happens none of the CSS / JS will load and the theme preview lacks stylesheets.

It turns the stock .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
RewriteRule .*\.git/.* - [F]
</IfModule>

into something much longer, including

RewriteRule ^assets/css/(.*) /wp-content/themes/roots/assets/css/$1 [QSA,L]
RewriteRule ^assets/js/(.*) /wp-content/themes/roots/assets/js/$1 [QSA,L]
RewriteRule ^assets/img/(.*) /wp-content/themes/roots/assets/img/$1 [QSA,L]
RewriteRule ^plugins/(.*) /wp-content/plugins/$1 [QSA,L]

It also adds the Boilerplate HTML .htaccess rules, and removed my .git rule; I’m going to add it back in for safety despite testing showing that the Git folders are not accessible.