Choose background colour, show bounding box

[johnjcamilleri]

The existing background controls are nice, but in addition I would really like to:

1. Select an arbitrary colour for the background
2. Toggle a border around the bounding box of the preview

This was quite easy to achieve with some custom CSS:

.svg-preview svg {
  border: 1px dashed black;
  background: white;
}

But some controls would be handy!