How to change the GUI theme on Xinit / X11 / X display / Xorg?
X doesn't have a theme because it's not responsible for how anything looks. Well -- that's not quite true; cosmetic preferences can be set for very old school X apps using a configuration file, but themes are not a concept there.
Global themes are commonly implemented independently by:
The window manager, which controls the appearance of window frames and titlebars.
The desktop environment (DE) -- if any (you aren't using one) -- which controls things like taskbars, and may provide a unified interface to the window manager.
Widget libraries, which are used by individual applications to provide normal GUI features like menus and other controls. DE's also sometimes provide a unified interface to one or another of these. Since they are are coded into the applications, there may be more than one in use on any given desktop (resulting in a subtle or not so subtle difference between one app and another). The two most significant ones on GNU/linux are Gtk+ (primarily) and Qt, both of which use incompatible major numbering systems, meaning, e.g., an app written for Qt 4 can't use Qt 5 and vice versa.
This last point about versions is significant because the theming for each major version is controlled separately, and you could have contemporary apps using Gtk+ 2, Gtk+ 3, Qt 4, or Qt 5. I know that epiphany uses gtk, but newer versions might use v. 3.
Gtk+ 2 is controlled with a per user file,
~/.gtkrc-2.0. An easy way to set that from the desktop is
gtk-chtheme, which is an independent package:
apt-get install gtk-chtheme
Then just run it. There may or may not be many themes installed on the system; there will be more available if you look through:
apt-cache search gtk | grep theme
Installing them will make them available right away in
how I could eliminate the outer border by not using / using the matchbox manager?
If all you are doing is starting one app, there's not much purpose to a window manager unless you want a frame (needed to resize the window).
Note that this:
while true; do
epiphany-browser -a --profile ~/.config http://google.com &;
exec matchbox-window-manager -use_titlebar no;
Finishes right there -- the next line,
sleep 2, never happens. This is because
exec replaces the current shell with the command. So all you've done here is start epiphany then matchbox. There's no loop.
X exits when the xinit script ends (which is why some people use a loop here), so in this case, when matchbox dies, the whole GUI will shut down. Using matchbox in this keystone position may or may not serve a purpose. If instead you used no window manager, just one line:
exec epiphany-browser -a --profile ~/.config http://google.com
Then X would exit when epiphany does. Notice no
& at the end there, or what would happen is epiphany would fork, the script would end, X would die, and epiphany would go with it.