fix: Improve types to include css variables

[dca123]

In the docs regarding css variables, it's recommended to cast it as any.

This PR should allow you to add css variables, i.e "--${anthingGoesHere}". If the initial "--" is not there, an error is thrown !

Right now it only allows for strings & numbers as the properties.

Valid

{
  "--myColor": "#eeeeee"
}

Invalid

{
  "myColor": "#eeeeee"
}