New GTK+ incompatible with old themes? How can I install new themes on an old system?

[Dan Stromberg <dstromberglists gmail com>]

Hi folks.

I'm attempting to run a "hello world" application copied from https://developer.gnome.org/gnome-devel-demos/stable/hello-world.py.html.en .  I did change it, but only minimally - specifically, I added a #!, and gave it a gi.require_version('Gtk', '3.0')

It runs fine displayed on a Linux Mint 18.1 system (as long as I don't monkey with the themes - trying to change themes using lxappearance breaks it).  It doesn't appear to run well displayed on a Linux Mint 18 system.

I used lxappearance to change themes.  I'm getting the feeling that either this was a mistake, or the version of lxappearance I'm using is too old.  I'm not using LXDE as my desktop, BTW, I'm using a mix of Cinnamon and fvwm2.

Some relevant files are in:

http://stromberg.dnsalias.org/svn/gi-hello/trunk

More specifically, at that svn URL:

* hello.py is the slightly modified hello world app from the gnome demos URL mentioned above.

* good.png is a screenshot of what the app looks like when things are going well (displayed on a Mint 18.1 system without having used lxappearance).

* bad.png is what the app looks like when things are not going well (displayed on a Mint 18 system, or a Mint 18.1 system on which I've tried to change themes using lxappearance).

Is there a good way of running a new GTK+ app remote displayed to a system with an old theme, or of running an old app remote displayed to a system with a new theme?  Do I just need to upgrade the old themes to something modern? And what tool should I do the theme upgrade with, if any?

I googled a bit, but it looks like a lot of the information around the internet on this topic is dated now.  If there's a manual or FAQ list I should look at, feel free to point me at it/them.

Thanks!