can't do custom dark themes in newest update?

whenever I try to use a theme that defaulted to dark mode in the previous update, it defaults to light mode instead. for some websites this means that it displays in bright white. i need dark mode. even when i set the toolbar colour (which previously controlled dark/light in Firefox Colour) to black it's on light mode. how do I get dark mode back, i hate this. this theme (picture attached) used to be dark mode but now it defaults to light. how do i get dark mode back.