Allow note colors for non-note files that use Set Syntax.

[rocketnova]

It's possible to use PlainNotes to set the syntax (Set Syntax: Note) for files not created using PlainNotes, such as a generic .md. But when you try to change the file's note color (Notes: Change Color...), you get an error:

Traceback (most recent call last):
  File "/Applications/Sublime Text.app/Contents/MacOS/sublime_plugin.py", line 1036, in run_
    return self.run()
  File "/Users/REDACTED/Library/Application Support/Sublime Text 3/Installed Packages/PlainNotes.sublime-package/notes.py", line 218, in run
ValueError: 'Boxy Yesterday' is not in list

where Boxy Yesterday is the Sublime Text theme.

This change sets "Yellow" as the fallback color so that changing the color doesn't fail.