The Numix gtk theme in 16.04

Several creators of gtk3 themes are using a format (for want of the correct term) that makes it more difficult to modify than previously possible.

Please note that I'm excluding Adwaita from consideration here.

gtk3 themes in 14.04 had a simple structure: gtk.css, gtk-widgets.css, an apps folder with app-specific css files and an assets folder with png (or svg) images. (I don't know what went on in 14.10, 15.04, and 15.10.) All the css files could be easily edited.

More recently, I can't say when, some gtk3 themes have a different structure. Even though you'll see most of the files and folders I mentioned above, there may also be gtk.gresource, and gtk.gresource.xml, a folder called scss containing *.scss files and more folders.

The point is that with such themes, you can't get away, IMO, by editing any .css file. Now, again IMO, the procedure to tweak anything is different and more complicated.

I'll stop here because I haven't gone there so far but thought I'd post this because of a thread started here: Can't Change Selected_bg and the poster mentioned Numix.

The much-talked about Arc theme has the new format as well.

Xubuntu 16.04 users have the shimmer-themes package installed by default. In that, Greybird is still conventional whereas Numix has the new format.