Please confirm my understanding:
Somebody installed Trisquel on your hard disk;
You installed GNOME Shell and successfully got additional accessibility
features (such as the zoom);
You believe you enabled something else in the "Universal Access" utility that
turned black part of your interface;
You could not reverse that change from the "Universal Access" utility;
You reinstalled some packages hoping the bad configuration would be reversed
(it is not, like onpon4 explained);
The Synaptic package manager froze;
You tried to download a new ISO of Trisquel from
https://trisquel.info/download but the download failed.
I imagine you have already tried rebooting and the black remains. Am I right?
If so, then you need not reinstall anything. Since you do not seem to care
about losing the current configuration of your applications, you can simply
create a new "Administrator" ("+" button in "User Accounts" in the "System
Settings", after "Unlock"ing the utility with your password) and log in with
it. All applications will be back to their default settings. You can then
copy your files from the previous user account to the new one (execute 'gksu
nautilus' in a terminal).
If you cannot use "User Accounts" (because of the black parts of the
interface), then you can log in a terminal after typing Ctrl+Alt+F1 (Alt+F7
will get you back to the graphical session) and add a new administrator
(named "banana" in my example) with 'sudo adduser --system banana'.
Now, if the black disappears after a reboot, then your hardware certainly is
defective and I would test the RAM with "memtest86+" (in Trisquel's
repository).
That said, I do not understand how messing up in "Universal Access" can
blacken part of the interface and I cannot find anything that relates to
"brightness" in this utility. And Synaptic never crashes here. That is why I
would actually test the hardware no matter what.
