Swap add and modify colors, update icon for Big Sur

[jason5122]

I swapped the colors associated with added files and modified files, as discussed in #703. This is because virtually every other Git client has this color scheme and it seems to make more sense than adding being blue and modifying being green. Unfortunately, it seems the icon file sizes are larger after editing them in GIMP (~300 bytes → ~3 KB) — I'm not too sure why this is the case. If someone is more experienced with this, please feel free to make changes!

I've also updated the icons for Big Sur using the icon set provided by @mdznr in #723.

Screenshot

Replacement icons

Normal

2x

I agree to the contributor agreement.

[lucasderraugh]

Thanks, @jasonhan-vassar! I think out of all the changes this color change one is going to be what I hear the most feedback from 😅. I think it is appropriate though and probably what most people would expect at least using GitHub. I'll review it today and get this 1.2 release out before for Christmas.