Allow to change bg colour of canvas

[kaelemc]

In cases where labs should be shared the lab designer might be using dark theme and have white text. If the lab is shared and the next person has a light theme, text mightn't be visible.

I think we should add to the lab settings panel, default to vscode theme, otherwise allow user to set bg colour and when we open a topo, read from annotations.json if the canvas bg colour is specified, use that.

[FloSch62]

I think we also could can have auto for text ( black/white depending on the theme )