<...>
Themes were around
back as early as 2011, though in 2014 themes other than the default one were already disabled. It seems that the code was difficult to maintain
as depicted here, although there are quite a bunch of post before then that verse on custom themes.
I figure themes had many more implications than we can see on the surface, and are not limited to just visual makeup. For example, the extinct ads on the forum all stated that:
<…> Ads appear only on topic pages with more than one post, and only for people using the default theme. <…>
See
this post as a sample.
This other post from 2011 already pointed to this fact.
Anecdote: Even
Satoshi left the default SMF theme for the Admin account in 2009 in case something went wrong modifying custom themes, which gives us an idea that they are non-trivial to manage, with broader implications than (could) meet the eye.
