Themes is a part of the Home Assistant
Themes is not enabled by default in HACS, the reason for that is that it is not enabled by default in Home Assistant.
Enable themes in HACS
There are 2 ways of doing this.
Enable if you used the UI to configure HACS
Add a checkmark to the left of themes in the UI configuration for HACS.
Enable if you used the YAML to configure HACS
If you used YAML to configure HACS, add this under your HACS connfiguration to enable themes.
When you have done that you need to restart Home Assistant.
Enable themes in Home Assistant
Add this to your
frontend:themes: !include_dir_merge_named themes
Themes download location
All the themes you download with HACS is stored in
themes/ in your Home Assistant configuration directory.