The accepted answer from olleicua (to be more specific, the comment from sammy) stopped working for me as of tmux 3.2.
I changed the line to the following and now it's working again:
set-option -g status-style bg=default
The "3.2 questions and comments" thread on the official github page explains why this is the case:
status-fg
and status-bg
are now applied before status-style
rather than instead of; this means that if either is set to default
the status-style
colour will be used. To fix, replace status-bg default
with status-style bg=default
.