On line 638 of CommandHandler.cs (in ContentPatcher) you declare the alpha channel of a Color to be a. The proper name of the property is alpha.

Fix:
use this for line 638:

alpha: pixel.A

The names are inconsistent between Linux/Mac (alpha) and Windows (a). The compiled version will work fine in both, but I removed the names to avoid compile errors on Linux/Mac. Thanks for reporting it!

No problem. Went through the process of setting up a local environment so I could provided a PR. Do you mind if I move the comment to above the declaration?

It's a bit odd here for a multiline statement, but I use comments on their own line to group blocks of code and on the side for a note about a specific line.