The package "trisquel-desktop-common-data" provides, among other files,
/usr/share/glib-2.0/schemas/x_trisquel.gschema.override that apparently
defines Trisquel's default look (including the layout of the panel, the
background, the icon theme, the default screensaver, etc.).
"mate-panel", I guess. It comes as a dependence of even
"mate-desktop-environment-core": you should have it. I believe your problem
deals more with the configuration of the panel. I use GNOME Shell and cannot
really help with that.
This metapackage installs more than just the theme.
There is "trisquel-gtk-theme" for Trisquel's GTK theme, "trisquel-ion-theme"
for Trisquel's icon theme, "trisquel-wallpapers" for Trisquel's wallpapers,
"plymouth-theme-trisquel-logo" for Trisquel's Plymouth theme, etc.
X11 is needed and it's not installed together with the desktop environment.
It is surprising that "xorg" does not come as a dependency.
Using the NetInstall to build your custom system is not that hard: you only
need to know the 'sudo apt install' command. What is harder is to decide
what to install and discover the related package name. Assuming you want
Trisquel's default desktop environment, you can execute:
$ sudo apt
