With the new AppGameKit release, the windows client comes with a few extra themes for your IDE, which enables one to change the IDE color-scheme from the default glaring white that burns ones eyes out when sitting in a dark room at five in the morning whilst overdosed on coffee chasing bugs in the code...
Not so with the linux client however. But alas, it is easy enough to get going. If like me, you have a dual boot win10/ubuntu machine, just pick up what you need from the win10 version of AGK. If not - or you want more themes - you can download a theme with the required files from this thread: https://forum.thegamecreators.com/thread/216961
Now, on the linux client, the files are organized a bit different. Under 'AGKLinux/Tier1/Editor/' there is no folder named 'data', which is where IDE themes are handled on the Win10 client. This is fine however. You'll find it all on linux under 'AGKLinux/Tier1/Editor/share/geany/'.
The theme files for the main editing window go into 'AGKLinux/Tier1/Editor/share/geany/colorschemes/' - once copied over, activate in the IDE using View->Colorscheme menu. To get the rest of the IDE looking lovely, you need to copy the folder 'themes' (from either the themes in the link I gave above, or the win10 version under 'AGK2/Tier 1/Editor/share/') into the 'AGKLinux/Tier1/Editor/share/geany/' folder.
Next you need activate the theme, by editing the 'geany.gtkrc' file. You need to change the last line from its default to: "./themes/<your desired theme folder name here>/gtk-2.0/gtkrc". Then save and start up AppGameKit in its new fancy dress.
Happy coding!
(themed up AppGameKit on ubuntu)