The last version of rtd_theme on the site.
Theme version is limited by sphinx-rtd-theme<0.3:

Despite the fact that #4096 has been already merged, our latest build used old theme, and in consequence html_theme_options parameter doesn't work.
Out conf.py is very simple and can be found here.
When the latest theme will be able by default?
Is it caused by that latest release was 19 days ago?
So the sites are built not from master branch, right?
What is in rel is what's deployed. When the site is deployed, master is merged into rel. Also you can always check the changelog to see what's deployed.
2.3.12 release fixed the problem. Thanks for the info and changelog link!