Improve the main icon

[bdcomp]

Thanks for the add-on!

The current icon is way too much bright and doesn't feel native part of Firefox.
I would say it's even hard to recognize the ruler from the current icon.

For inspiration, current Firefox icons may be considered:
https://design.firefox.com/icons/viewer/
They are black and bold.

Ruler 2.0.3
Firefox 67.0.2 (64-bit)
Kubuntu 18.04.2