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