Base16 themes for highlight by Andre Simons: http://www.andre-simon.de/doku/highlight/en/highlight.php
Copy the desired theme from highlight-themes/ to $(brew --prefix highlight)/share/highlight/theme/.
This assumes you installed highlight via Homebrew. If that is not the case, see section for Linux.
You'll find the correct spot to put them themes by inspecting highlight --list-scripts=themes for the config directories that are being searched.