Edit Scripts In Binary and Library Folders
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Yes, I know it is not the recommended way, but I wanted to
make some changes to my Thunderbird mail client: all I wanted 
was to have the background color of the chat tab's text area 
changes to the colors set in preference tab, just like the ink
color. But I found that when I choose white ink and black paper,
I cannot see anything I type in the chat because it's white on white.
With some recursive grep work in /usr/local/lib/thunderbird, I'd found
that omni.ja matches. What is that file? Well, with the command 
'file omni.ja', I found it is an archive of the famous Zip format used
by all operating systems.  
Now, how do you search inside a zip file? zgrep won't do! It just write
that 'omni.ja' matches.
I have 'zipgrep' but without a fix it won't do its job.
It yields "unzip: illegal option -- -"
Where is zgrep? It is in "/usr/local/bin".
What sort of file is it? a shell script! Wunderbar!
So, the illegal option is "-p-L". I found 3 lines with "unzip -p-L".
A space between flag did the job.

Now, to "omni.ja". I've found using the Thunderbird's 
"Developer Toolbox" (Tools->Developer Tools->Developer Toolbox")
that the CSS class of that text area is "cov-textbox".
Now, with the fieed zgrep I can find that the path to the relevant
CSS file is "chrome/classic/skin/classic/messenger/chat.css".
So, I change the CSS definition of the class as follows:

.conv-textbox {
  padding: 2px;
  box-sizing: content-box;
  border: 1px solid transparent;
  background-color: inherit;
}

Now, the background color is the selected one.

I've sent the fix to newsgroup "mozilla.dev.apps.thunderbird". I hope
to see a fix pretty soom.