Bug: custom CSS does not work

I’m trying to connect a custom stylesheet to the specific site or to all sites in OmniWeb, and it doesn’t work. Any custom CSS file doesn’t do anything with any site with any settings. I tried the same CSS that worked with Stylus addon in Firefox and Opera, and the most simple CSS also.

macOS10.14.6, OmniWeb 6.0 test (v632.0.114)