A completely dark variant (dark GTK3 + dark GTK2) has been available for approx one month now (see Github link). The light theme will respect the Global Dark Theme option too, but this won't make GTK2 app dark ofc.
You have to edit `common/gtk-3.0/<your-gtk-version>/_colors.scss`. To disable the transparency you have to change
$header_bg: transparentize(#e7e8eb, 0.05); to $header_bg: transparentize(#e7e8eb, 0);
$dark_sidebar_bg: $osd_bg_color; to $dark_sidebar_bg: opacify($osd_bg_color, 1);
After that you have to regenerate the css files. There is a readme in the same directory that tells you how to do that, but in short you have to install ruby and execute `gem install bundle && bundle install` from the same directory. Then you have to run `bundle exec sass --update --sourcemap=none .` Now you can reinstall the theme. Make sure you follow the instructions in the README.md closely, since the installation process has been changed recently.