If seems to be disabled deliberately:
Change Theme. At least that is what the thread indicated back in 2016, indicating that it had never been enabled (which I’m not sure is true if we go back further in time).
It was something considered is a list of forum specs back in 2011:
new forum software specificationsUI
The default theme should be minimalistic like the current theme. Nothing that looks "web 2.0": no speech bubbles, no significant space between posts, no significant hover effects, and few rounded corners. Use even fewer images than the current theme: no image buttons.
The default theme must work well with all functionality and a reasonable page layout on text browsers without JavaScript. It should also work perfectly on browsers with unusually small browser dimensions. It should be at least somewhat usable (though maybe not pretty) on ancient and broken browsers like IE6.
Same color scheme as we have now: light with some blue.
There should be functionality for allowing users to choose from among several UIs. You only need to provide the default one, though.
Admin settings that are not changed very often can be made changeable from files instead of from a web interface, though changing the settings from the files should be easy. The web interface must not allow admins to add/edit UIs, execute arbitrary code/SQL, or tamper with logging.
It seems that there was an alternative theme is 2012 called Blackbox:
I have added the much-requested "watchlist" feature. It's like "show new replies to your posts", but it only shows topics that you've chosen to watch, and you needn't have posted in the topic. I only added it to this forum's default theme, the SMF default theme, and BlackBox.
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Meta has some threads on custom solutions to change the theme, such as:
List of Bitcointalk.org Userscripts/ Add-ons improvement next dark theme.
Script to Change Bitcointalk Background ColorBitcointalk Dark Theme 
