I’ve just checked your original post. You state that you’re using a Pinebook Pro. That’s an aarch64 processor, and maybe gtk3-nocsd is avalaible in the official repositories for aarch64, but it’s not in the repositories for the x86-64 platform.
I suspect your Emacs problem may be due to a missing package in the aarch64 repositories.
Are you so sure that Emacs is GTK3-based? I would rather be inclined to think it still uses GTK2. 
Emacs switched to Gtk3 a long time ago as far as I know. Maybe one of the first applications to switch. Just to your information I checked out the “About” info from the emacs I installed on my Sway Environment. It is stating
"Gnu Emacs 27.1 (… GTK+ Version 3.24.22, cairo version 1.17.3).
Gtk2 is dying and nearly all Gtk2 based software are rushing to switch to Gtk3. Lxde is the only desktop environment staying with gtk2 but it will likely end with gtk2 since the lxde is also transforming to lxqt slowly.
Ok, it makes sense. Not all packages in aarch64 is available in amd64 and vice versa. Actually, I have that gtk3-nocsd package (version 3.0.1 or similar) already installed in my Sway Environment.
Manjaro KDE development team apparently do not consider emacs integration as otherwise, these issues like missing dependencies (gcc) or missing icons on emacs woould not happen.
On Pinebook Pro, I do not have much choice, the only basic working editions are KDE, XFCE, and Sway. I wish there were more options but I seem to pick one from the three. KDE is not aligning with my workflow due to lack of support for emacs. Xfce is not stable enough (I ended up XFCE consuming 2.5gb of memory after two updates). So, Sway seems to be my best bet.
Sway is lighter than KDE so I should see the merits of staying with Sway. And Emacs work nicely in Sway Edition.
You should try installing a GTK3 theme and applying it in the system settings. I believe that will solve your problem. KDE supports emacs just fine btw, if you’re willing to make a little more effort to solve the issues. No distro/DE will have everything working out of the box.
Ok, I might give this a try. Yes, I agree KDE supports emacs fine because I used it in Kubuntu. It was feeling like a native KDE/QT application almost.
I am assuming you also have emacs with no icons on the toolbar since you’re on Manjaro KDE. It is somewhat puzzling because Emacs seems to pick up the KDE theme but skipping the icons. So it may not be a complete theming issue.
I am ok with Sway as of now. So, I will try Emacs in Manjaro KDE will be for experimenting. If I could get something similar to what I had in Kubuntu, I can consider the switch to KDE.
Hi. I was a Manjaro user but now I’m using EndeavourOS.
Anyhow I have the answer to your questuion because I had the same problem:
I recently switched from Cinnamon to Plasma. Everything was fine but all the icons in the emacs toolbar disappeared. I found a solution. To whom may be concerned, here it is:
you have to put the following instruction in your .emacs configuration file (e.g. .emacs.d/init.el):
(setq tool-bar-style 'both)
or (if you want only icons in the toolbar)
(setq tool-bar-style 'image)
Hoping to help all emacs and plasma users.
Kind regards,
Roberto