EPUB files are just ZIP files containing HTML documents and images so a possible dark theme solution is simply unpacking them using the unzip command and opening the HTML files using the browser (which supports a dark theme).

Wrapper script:

>cat bin/epub.sh
#/bin/sh
unzip -qcp "$1" "*ml" "*.htm"   | lynx -force-html -dump -stdin -nolist | less -r  -M +Gg

With a few tweaks it's possible to use a tmp directory with mktemp and read
epubs with images with something lightweight such as dillo.

First install Openbox packages, a dark GTK theme and a theme switcher application:

Then run lxappearance and set the dark theme. In order for the new theme to work properly you might need to log out and log back in.

Qt applications can also use the GTK theme. First install the following package.

You'll also need to install the package qt5-styleplugins (from AUR) so that Qt can use GTK themes.
Then add the following line to the file .bashrc

After logging out and back in, Qt applications when run from the terminal should use the dark theme.

In order to edit the items in Openbox's menu, it's best to install a menu editor. I installed obmenu3 from AUR.

When a Qt application is run from Openbox's menu it will not use the dark theme unless the execute command is altered e.g. via obmenu3. A command such as this one for Avidemux works (just replace avidemux 3_qt5 with the command needed to run the Qt application).

If you want a task manager as well you can install tint2.

In order for it to run after login add the line

to .config/openbox/autostart

Qt applications when run from tint2 will not use the dark theme unless you edit the .desktop files.
Copy the relevant files to .local/share/applications (you might need to create the latter directory), e.g. for Avidemux.

Then edit the Exec line - I use this one.

What's still left to do: get PDF and EPUB files to use a dark theme (using mupdf and epdfview).