HACS will try to match your Home Assistant theme as much as possible. There are also several variables you can use in your themes.yaml file to theme HACS further:

Variable	Usage
hacs-badge-color	Controls the background color on the "NEW" badges in the store.
hacs-status-installed	Controls the icon color for installed, up-to-date repositories
hacs-status-pending-restart	Controls the icon color for installed repositories that are awaiting a Home Assistant restart
hacs-status-pending-update	Controls the icon color for installed repositories that have an update available
hacs-status-not-loaded	Controls the icon color for repositories that is not loaded
link-text-color	Controls the color of hyperlinks

Here's a basic example of customizing one of these variables in themes.yaml:

hacs-badge-color: "var(--primary-color)"