Is there any kind of theme editor, so I could make my own theme or alter an existing one?
no
Themes are only a particular combination of appearance settings: you can save those setting to a file (.scrtheme) and reload them very simply.
Choose the theme (from the Windows > Themes menu) which is closest to your conception of your new theme. Then open the Options dialogue (F12) and go to the Appearance tab, and make the changes there. You may find a couple of other relevant settings scattered around, but they’re mostly in Appearance.
Test your changes out and once you’re happy, F12 again and choose ‘Manage’ in the bottom left hand corner, then ‘Save theme to file’ .
Your settings will persist anyway, but once you’ve saved them, you’ll be able to reload the new theme any time from the Themes > Load theme from file menu.
HTH.
I don’t think that’s true, at least not entirely. You can change most of the editor, notes, binder, etc. but some of the elements like the menus or metadata panels can’t be changed that way. But maybe I’m wrong…
That’s partly why I suggested you start with the built-in theme nearest to your intended design. You can change the colour of over 50 elements, so it’s a bit unfair to say there isn’t a way of editing themes. Whether the ability to add the colour of the menus will be in later versions, I don’t know, but it’s not a bad first step, I think.
The OP question was: “Is there any kind of theme editor…”
There isn’t.
Yes, you can modify some of the options of the existing themes, but that’s not like to create a new theme were all window colors and fonts are in sync.
Regards,
M
The OP wanted to know how to change the themes. Now they know. Whether they choose to call that a theme editor because it can’t change 3 elements out of sixty or more is up to them. I do call it an editor, because it allows me to edit the themes, but you call it what you want.
Arguing about this is pointless.
Thank you for your willingness to help. I appreciate it.
I hope the OP will find their answer somewhere among the lines. 
Regards,
M