That’s good - and I had already voted for Reminders integration.
What’s interesting about a date-based formatting in the theme is that you would have very much control over what exactly “that thing” is in “make this thing red”. Personally, I’d prefer it if only the tag changed its color, not the whole line.
What if you could define past, today, soon, and future like saved searches, e.g. @tagcolor(today, green) or @taskcolor(past, red)? Maybe this could be expanded to other data strings: @taskcolor(“Boss”, orange)? Then the theme wouldn’t be needed to deal with emphasis based on data at all.
Just a quick idea.