Refactor the themes part

So currently if someone wants to create a theme they have to go to the card.style.mjs and add a class with name of the theme and set the css variables.

This is super useful and makes things extremely easy to create themes but as we go adding themes we go on increasing the numbers of lines in the JavaScript string (the css is just javascript string). It would be helpful if there is any other way to handle this since I don't want to go on making the response size larger and larger.