This thread is a few years old but is still the one that comes up as the best search result, so I'm answering with what finally worked for me. This is based off of tmux FAQ.
...but the instructions aren't completely clear on when or where to substitute the -256color string. I use gnome-terminal (v 3.16.2) with tmux, and this worked for me:
$ mkdir $HOME/.terminfo/
$ screen_terminfo="screen-256color"
$ infocmp "$screen_terminfo" | sed \
-e 's/^screen[^|]*|[^,]*,/screen-256color|screen with italics support,/' \
-e 's/%?%p1%t;3%/%?%p1%t;7%/' \
-e 's/smso=[^,]*,/smso=\\E[7m,/' \
-e 's/rmso=[^,]*,/rmso=\\E[27m,/' \
-e '$s/$/ sitm=\\E[3m, ritm=\\E[23m,/' > /tmp/screen.terminfo
$ tic /tmp/screen.terminfo
And tell tmux to use it in ~/.tmux.conf:
set -g default-terminal "screen-256color"
Note: I tried it once without the -256color and since that didn't work (still seeing italics instead of highlighting), I had to delete everything under the .terminfo dir (another dir called 's') before the infocmp would work.