This is also an issue with Adwita-dark. Certain UI Elements just don't seem to follow any sort of theming. In older versions of Ckan I believe I just used GTK_THEME= and set Ckan to a light theme to let it even be usable however it doesn't appear that Ckan follows this anymore Using the GTK2 method doesn't work either.

The GTK2 way worked for me:
$ env GTK2_RC_FILES=/usr/share/themes/Breeze/gtk-2.0/gtkrc ckan
(substitute Breeze for your light theme of choice)

Still would be nice if it properly worked w/ dark themes, but most mono apps seem to be broken in this regard

Same problem here with Adapta-Nokto theme. The Orochimarufan workaround has no effect in my case (tested with Adwaita light theme)
OS: Ubuntu 18.04 x86_64
CKAN 1.25.3
Gnome 3.28.3

Can someone please post installation/setup instructions for Arc-Dark, Adwita-dark, or Adapta-Nokto? The dark theme I use is available via apt install numix-gtk-theme, then using the old (and now removed!?) "Global Dark Theme" toggle. I don't see APT packages by these other names.

EDIT: Nevermind, Adwaita-dark is already installed!

• The menu bar has no color properties set
• The toolbar has BackColor set to SystemColors.Control
• The mod list has BackgroundColor set to SystemColors.Window

It looks like Mono's ToolStrip (and MenuStrip which inherits from it) may ignore its BackColor property; it always passes SystemColors.Control to ToolStripRenderEventArgs.BackColor instead of its own BackColor:

https://github.com/mono/mono/blob/f0921fd850b0c8e61d5954fa71471189d2a55447/mcs/class/System.Windows.Forms/System.Windows.Forms/ToolStrip.cs#L1021

That could be both the cause of the problem and a barrier preventing it from being fixed in CKAN code.

Submitted mono/mono#10942.

Also submitted mono/mono#11196 to fix message boxes, but we only use that in the notification of game update shown in that screenshot.

Submitted mono/mono#11287 to really fix the toolbar backgrounds. See that link for the gory details.
In the process of investigating that I think I also discovered a way to do a workaround in CKAN code, stay tuned...