I had never touched the Irssi default theme before, I'm very happy with it. I've been using adv_windowlist for a few years now to manage the many channels I'm in. And recently I switched to a new terminal that apparently supports more colors than my previous one, and adv_windowlist had become less readable.
Anyway, I wanted to customize the colors of adv_windowlist, the "no change" status in particular. It was a light grey, and the "active" status was white, far too similar. It turns out adv_windowlist uses the colors from the theme, and the "no change" status uses the one from sb_act_none (or falls back to other values if that is not defined).
The solution Nei pointed me towards was to pick a valid color and to add it to .irssi/default.theme.
sb_act_none = "%p$*";