When I right click on the file, there is no "Open with.." item in the context menu, so indeed, there appears to be no way to open that file in a text editor using Nautilus.
To reproduce, you don't need theme. Any text file named foobar.theme will display the same behaviour.
I can confirm this.
To reproduce, I tried with the Ubuntu Smooth Orange theme's index.theme file (http:// www.gnome- look.org/ content/ show.php? content= 37533 ). Nautilus displays that icon as "Human" and cannot open the theme file.
I have attached a screenshot of the same.
When I right click on the file, there is no "Open with.." item in the context menu, so indeed, there appears to be no way to open that file in a text editor using Nautilus.
To reproduce, you don't need theme. Any text file named foobar.theme will display the same behaviour.