Check out these threads that include guides. That should get you started and if you run into an issue you can post your questions on the corresponding thread.
A while ago, I created card-mod, a custom card* that lets you change various css options for other lovelace cards.
Much more skilled people than I have used that to create beautiful and awesome things.
Today, I’ve added theme support to card-mod.
Your ordinary theme file can now be used to
Give cards a flashing border
Make that border only flash when a certain entity is in a certain state
Make fan icons spin when they are on
Change the border color of the badges at the top of the screen dep…