I just checked gnome-look today and found many new GTK themes that just got uploaded recently. If you need some new themes for your Linux desktop, here are some that look good in my opinion:

Boje

This theme was uploaded at the end of last year and was rated very high. It is a dark theme with gray as the text color and red as the highlight color. Click here to check and download Boje.



Plane GTK

Plane GTK is another dark them worth-checking. It was uploaded after Boje but got rated even higher. Every detail of Plane GTK, from the text font to the scroll buttons, was perfectly designed. Click here to check and download Plane GTK.


MediterraneanNight Series

The MediterraneanNight series is from the same designer who created the famous Adwaita cupertino, so it is no surprise that the theme got rated extremely high after just a few day it got uploaded. Definitely a very worth-checking theme. It got both dark and light versions. Click here to check and download Mediterranean Night Series.



Silva Flat

For those who prefer light, minimal and stylish themes, Silva Flat will be a great choice. It just got uploaded 10 days ago but already has a very high rating score. Click here to check and download Silva Flat GTK theme.



SimpleX

SimpleX is another nice light theme that's worth-checking. Click here to check and download SimpleX


Also, here is a collection of some dark GTK themes I like.