In lots of terminals, it's possible to detect the background colour of the terminal with something like this:

#!/bin/bash

oldstty=$(stty -g)
stty raw -echo min 0 time 0
printf "\033]11;?\033\\"
result=""
while [ -z "$result" ]; do
    read -r result
done
stty "$oldstty"

echo "$result" | sed -E 's/.*(rgb:[a-f0-9/]{14}).*/\1/'

Then you get output like rgb:0000/0000/0000, rgb:ffff/ffff/ffff, rgb:158e/193a/1e75, etc.

Textual could use this to select a dark or light theme by default. This would be nice as apps would behave and look as users expect them without any configuration.

I would prefer this to happen automatically but, if you think this is not good as some terminals don't support this, then textual/rich could at least provide code to detect this.

I mention it in https://discord.com/channels/1026214085173461072/1033752510848053399/1075543323516866662 a while ago, but then forgot to open an issue.