It seems that as of version 2.2 a new set of UI icons have been created for SourceTree for Mac.

Please consider improving the new UI icon set both in the toolbar and in the sidebar as well. The new line icons virtually merge into the void on a 2560x1440 resolution screen.

When you open a Finder window and look at the icons in the sidebar, you can see that there is good contrast between the icons and the background. In addition, the icons in the Finder window are not line icons.

Due to their line weight and their color, the new line icons in SourceTree are hard to see and (especially on the sidebar) gives the illusion as if they were inactive (aka grayed out). But icons on the sidebar that are active have the same light gray color as well which makes it hard to distinguish. Also, there's enough space on the toolbar to make the icons bigger.

Personally I am not fan of the new UI icon set AT ALL. I have no understanding of why you guys had to remove a perfectly fine and nice looking icon set and replace it with a mediocre one.