Canvas dark mode

Please submit only one feature request in one issue!
It would very useful to apply a dark theme to the canvas.

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
This could be achieved by inverting the background color and the strokes colors. Since these functionalities are already implemented, there's just the need to call both of them together.

Describe alternatives you've considered
I think it's interesting to have:

1. a button to manually toggle the dark mode, independent of the user theme;
2. an option on the settings to toggle the dark mode according to the user theme (light or dark)

Additional context As i mentioned previously, this is achieved just by inverting colors of the strokes and the canvas background: