It's just kind of fugly and limited compared to other PHPBB dark themes, and the really awkward sidebar on the right has been fucked since forever which leads me to believe it's probably never ever going to be fixed. Also the fact that we can't but a cool banner like Blackgreen because it only allows for tiny logos.
I was thinking of using https://www.phpbb.com/customise/db/style/ad_infinitum_2 as a replacement, but it hasn't been updated in 12 years and it's on a super old version of PHPBB, so IDEK if it will work. If it doesn't you can just use the regular Black theme, or Blackboard Max to at least get rid of the broken sidebar.
