[Users] GTK3 CSS Term(s) for Message Part Icons?

[Dustin Miller]

For the developers (or others who might know), would you be able to
tell me what the GTK3 term(s) is/are for the message part icons that
show up on the right side of an opened message? Or pointers on how I
can figure that out myself?

I've run into a situation where I've upgraded my OS (Linux Mint Debian
5 to 6) and with the theme I was using (Greybird Pro) the box outline
that indicates which message part is selected is no longer visible.
(Not the end of the world, but unhandy.)

So far I've gotten to the point where I can confirm that the same issue
is present with the default theme in LMDE 6 ('Mint-Y-Aqua'), and that
when switching to the 'Mint-Y-Dark-Aqua' theme, the issue is gone.

So I tried to compare these two themes' gtk.css files to see if I could
find the difference that was causing the different behavior, but there
are a lot of lines of settings and I have no idea what GTK3 component
I'm looking for.

So I thought I'd try the list to see if it can save me some time in
figuring this out. 😀️

Thanks,
Dustin