When I run Kstars in iinux (Sellarmate) I get a dark background colour theme for all the windows. I'd like to do that on Kstars running on my Mac but there are only 2 themes, default and Mac, and they are very similar and have a white or light grey background.

Also, the colour "schemes" are not the right colours. The night vision one uses pink rather than red, and the classic has lime green lines and earth shadow :-s I suppose I could customize this but I'm pretty sure these colours are not what was intended.

So your issue is actually pretty straightforward. I assume you had KStars installed before the Themes and Schemes were updated so you have older files in your kstars data directory. The KStars data directory is in ~/Library/Appplication Support/kstars. You can copy the new files into this directory from inside the app bundle so that you have the latest schemes and themes in your data directory. If you right click on the KStars.app bundle and "show package contents" then navigate to /Contents/Resources/data you will find the default data directory. The folder you want to copy is called "themes." The color schemes are also in this directory but they don't have their own subfolder. They are files that end in .colors such as classic.colors. Just copy and paste and it should work.

Thanks @rlancaste! Looks nice!

Sorry to be a bother but there's another thing I'm trying to get working. I want Kstars/Ekos/Control Penel/etc. to all be tabs in one window. I can get this if I launch Kstars, then make it full screen (shift-cmd-F) and then start Ekos, etc. But I can't seem to make it this way by default. In the preferences, there are setting to make certain windows independent/dependent but often that does nothing except force some windows to be in front of others.

The "tabs in one window" feature is not a KStars feature but an OS X feature for Applications with multiple windows that are full screen. It is not a part of KStars code. This code does not run for programs that are not full screen. I believe that getting tabs the way you are looking for when it is not full screen would require a major change to the way kstars windows are handled.

The Independent/Dependent options for the KStars windows allow you to change whether the operating system considers the window to be a separate window or one that is dependent on or a child window of the other window. Yes one of the results of making the window independent is that you can then put it behind the skymap. Another, at least on OS X is the ability to use the feature you described. I don't believe this feature works if its a dependent window. When the window is dependent, it won't go behind the skymap.