Top left card is a custom:button-card which switches the theme just by clicking on the button (behind the scenes it’s using an input_select combined with an automation for frontend.set_theme).
Underneath is a count of certain domains like total sensors, automations, scripts, etc. Besides just displaying statistics, these have “secret” actions associated with them - for example holding on the scripts icon will perform the script.reload service, same for automations, etc.
Then under that is a glance card with some stats. These also have tap_action and hold_actions on them. Holding on the database icon will call recorder.purge service keeping the last 3 days. Tapping on the log icon navigates to /dev-info/ page, and holding on it performs system_log.clear service. hold_action for Uptime icon performs homeassistant.restart service, and tapping on version icon navigates to /hassio/dashboard so icon doubles as a shortcut to the update screen.
My other system monitoring stuff like processor and disk use is in a standard entities card for now, i plan to overhaul that soon though.