Don't require pre-existing background image

[leviem1]

Is your feature request related to a problem? Please describe.
Users might not want to use a pre-existing background image, and new users might be confused by this, expecting the plugin to handle this automatically.

Describe the solution you'd like
Add some simple settings with sane defaults to allow a background of a specified size and color to be automatically generated. The settings should be ignored if a background image is used instead.

Describe alternatives you've considered

• Low effort win: Ship a default background image somehow

Additional context
N/a