Sadly, this is not as easy as you’d think it would be.
You’re going to want to install card-mod
thomasloven/lovelace-card-mod: 
Add CSS styles to (almost) any lovelace card (github.com)
It can be done manually but, if you’re likely to want other custom elements in the future, you should probably install HACs (if you’ve not already) and use that to install card-mod.
Home Assistant Community Store | HACS
Then go look at at the card-mod docs + this thread
Card-mod - Add css styles to any lovelace card - Share your Projects! / Dashboards & Frontend - Home Assistant Community (home-assistant.io)
Particularly this post:
Card-mod - Add css styles to any lovelace card - Share your Projects! / Dashboards & Frontend - Home Assistant Community (home-assistant.io)
Have fun!