enhancement: custom background

### Is your feature request related to a problem? Please describe.

Adding a custom background to the card.

### Describe the solution you'd like

We can add a new feature to add a custom pre built background to the card, with the user having options to choose from.
There could be different types of background such as textures, gradients, and so on...

### Additional context

We can add a new API parameter: `&custom_bg=` to add the background.