Yes, I was thinking about using a greyscale icon which does not stand out so much. But note that it will still look out of place when using themes since there currently is neither a way for addons to specify themed icons for page actions nor to use the browser's foreground and background color as native widgets do. The relevant bugs for that are: https://bugzilla.mozilla.org/show_bug.cgi?id=1367042 and https://bugzilla.mozilla.org/show_bug.cgi?id=1409107

And I'm afraid you'll have to resort to copy and paste when using an offline reader since it is impossible for web extensions to launch arbitrary user-configured executables.