Ok this is really cool!

Found an unrelated bug with the Console UI while testing this. Happens when leaving the path filed empty when adding a new game instance:

Unhandled Exception:
System.ArgumentException: The specified path is not of a legal form (empty).
at System.IO.Path.InsecureGetFullPath (System.String path) [0x00025] in <9969b53fa7824abc872802c31d2374b0>:0
at System.IO.Path.GetFullPath (System.String path) [0x00000] in <9969b53fa7824abc872802c31d2374b0>:0
at System.IO.DirectoryInfo..ctor (System.String path) [0x00006] in <9969b53fa7824abc872802c31d2374b0>:0
at (wrapper remoting-invoke-with-check) System.IO.DirectoryInfo..ctor(string)
at CKAN.ConsoleUI.GameInstanceScreen.pathValid () [0x00031] in <30624ca0a0a44ac58f64c7b4f4f2cf39>:0
at CKAN.ConsoleUI.GameInstanceAddScreen.Valid () [0x00009] in <30624ca0a0a44ac58f64c7b4f4f2cf39>:0
at CKAN.ConsoleUI.GameInstanceScreen.<.ctor>b__0_0 (System.Object sender, CKAN.ConsoleUI.Toolkit.ConsoleTheme theme) [0x00001] in <30624ca0a0a44ac58f64c7b4f4f2cf39>:0            

Fix should be as easy as this:

diff --git a/ConsoleUI/GameInstanceScreen.cs b/ConsoleUI/GameInstanceScreen.cs
index d9f55bc4..555a0b1e 100644
--- a/ConsoleUI/GameInstanceScreen.cs
+++ b/ConsoleUI/GameInstanceScreen.cs
@@ -94,6 +94,12 @@ protected bool nameValid()
         /// </summary>
         protected bool pathValid()
         {
+            if (string.IsNullOrEmpty(path.Value)) {
+                // Complain about empty path
+                RaiseError("Path cannot be empty!");
+                SetFocus(path);
+                return false;
+            }
             if (Platform.IsMac) {
                 // Handle default path dragged-and-dropped onto Mac's Terminal
                 path.Value = path.Value.Replace("Kerbal\\ Space\\ Program", "Kerbal Space Program");

Do you want to add it to this PR? Otherwise I can do a separate one.

Otherwise this looks good. I thought about adding a way to make the theme choice permanent so you don't have to add the argument yourself every time, and to help those launching the Console UI via the system menu entry from #3052. A new field in config.json?

Interestingly, that exception doesn't conform to the .NET documentation; ArgumentException is only supposed to be for invalid characters:

https://docs.microsoft.com/en-us/dotnet/api/system.io.directoryinfo.-ctor?view=netframework-4.7.2#exceptions

I'm thinking this should catch all exceptions and treat them as failure...

For a persistent setting, ConsoleUI currently has none of its own, and I'd prefer to keep it that way.
How about an environment variable?

EDIT: Now you should be able to affect the system menu entry by adding this to your environment (presumably ~/.bashrc, but maybe ~/.kde/env/ckan.sh):

export CKAN_CONSOLEUI_THEME=dark

Yeah, after writing the comment it came to me that we shouldn't put UI-specific settings into the core's config.json.

Environment variables are a good idea. I kinda hoped that CommandlineParser would support them already, but doesn't look like it. There's an open PR for it though: commandlineparser/commandline#698

I kinda hoped that CommandlineParser would support them already, but doesn't look like it.

Same, this made me miss Python's click.

Hold on, the "no such theme" message should print the valid options. Gimme a minute to add that...