UI: Change default UMF colors for better contrast

[maxwxyz]

Default colors of the new unified measurement tool are hard to see on either light or dark themes:

This PR sets line and background color to a green which is visible on most themes. Contrast ratio between annotation text and text background is 13.65:1

After:

[chennes]

Others from @FreeCAD/design-working-group OK with this change?

[obelisk79]

As a default this is likely ok. However using primary shades of colors is not a great idea. Are these colors configurable by themes?

[maxwxyz]

yes you can configure them, I've an issue in OpenTheme about this.

[pierreporte]

The measurements are perfectly readable with the default theme. Is this an issue for the other official themes?

This being said, considering that the color can be set by the theme, I think that we should chose the right color for all of them. Green looks good, especially if we turn all sketch dimension green (as I proposed on other issues).

[Reqrefusion]

Green is suitable, but is this green really appropriate? It would be better if one of the Chameleon colors specified in the current design guide was used. #73d216 seems suitable for this job.