Windows AIO themes
It is possible to change the Gramps theme on the Windows AIO.
As of this writing, the AIO uses Gtk 3.18, so a theme MUST be selected that supports that version (themes that are designed for a later version may not work).
One choice that provides a pretty good looking dark theme is https://www.gnome-look.org/p/1206053/ the "OlliFri Gtk 3.18 Dark Blue Gradient".
Download the theme file (often a tar.gz) and if necessary decompress it using 7-zip of similar.
The decompressed files should be stored in "C:\Users\you\.themes" (replacing 'you' with your user name) so that the final directory structure looks like:
C:\Users\you\.themes Ollis-Gtk-3.18-Dark-Grad Gtk-2.0 Gtk-3.0 ...
Find your AIO installation and edit the file in "C:\GrampsAIO64-4.2.6\etc\gtk-3.0\settings.ini".
Add in a line that says "gtk-theme-name = Ollis-Gtk-3.18-Dark-Grad" (or whatever your theme name is). The complete file should look like this:
[Settings] gtk-theme-name = Ollis-Gtk-3.18-Dark-Grad gtk-xft-antialias=1 gtk-xft-hinting=1 gtk-xft-hintstyle=hintfull gtk-xft-rgba=rgb
Note: when you start Gramps, your console may show some "Gtk-WARNING **: Theme parsing error" messages. These are usually harmless, but if there are a lot, and your Gramps screen doesn't look right, then your theme may not be compatible with the version of Gtk Gramps AIO is using.
You can check the Gramps Gtk version by starting Gramps from the console;
Gramps Settings: ---------------- python : 3.5.2 gramps : GrampsAIO64-4.2.6 gtk++ : 3.18.9 ...