This concerns
* the C-API for theming which has effectively been removed preventing themes like Oxygen to work anymore.
* the CSS-API which broke various themes like Breeze till the devs adjusted it to work with the new API.
The last point might be the reason for you issue.
]]>The .css file is in the same directory as the executable
So it's not packaged/installed, just built and running locally?
Have you tried putting the css in ~/.config/gtk-3.0/gtk.css
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