Hard-coded background color for outside properties box

[indyteo]

Hello,

As we can see here:

arrows.app/libs/graphics/src/lib/PropertiesBox.ts

Line 103 in 73abfd3

ctx.fillStyle = 'white'

The background color for the properties box (in outside mode) is hard-coded to "white".

If we take for instance the "relationship types" theming options, we can modify the type-color and type-background-color. For the "properties", we only have property-color and no property-background-color, and the value is hard-coded to white instead.

It seems like this option was forgotten, because there is no logical reason to me not to have it. It makes the result looks very ugly especially in a dark theme...

I think this issue does not require a lot of workload, and I can make a PR if you want, let me know! 👍

Otherwise great tool, very useful, well done!