I've used the Highlights theme, designed by Nicolas F, for a while now. I tend to use dark themes on my machine, however, and the icons are not very visible with a dark UI.
So I went in and created a version of the Highlights icons for dark themes (mainly replacing dark gray with light gray). After consulting with Kovid and Nicolas, I want to make this variant available for everyone.
Since these icons are derivatives of the Highlights set, I imagine they fall under the same license terms: Google icons are under the CC BY 4.0 license and Austin Andrews icons are under the SIL Open Font License 1.1
Enjoy!
Edit 02/13/2020: Version 2 of Highlights-Dark is now out! It took a little longer than expected, but now includes all icons in the latest regular Highlights set.