*edit: it was late and I was throwing ideas. I made a solution that actually works in another comment.
I'm thinking you could have separate color profiles with a script that to change the file using symbolic link?
(I don't know what I'm doing, I'm a figure it out as I go type, but maybe it'll work. Please help!)
You'd need something like shizuku (allows shell operation permission) or root to be able to read the system dark mode status.
Using shizuku you could call it by
rish -c 'cmd uimode night'
Which would show
Night mode: yes / no
If you did that you could us an if statement (I don't really know how to write that out confidently at the moment)
And use that fuction to change the symbolic link to the color.properties file instead of the at a specific time I used below.
So you keep 2 (or more) color profiles in separate files and call those locations to link to based on time of day / uimode.
You could put the script in the .boot (to automatically start) or .shortcut (to start with the widget) Or maybe add the command to run the script in the termux bash_bashrc or profile file to have it execute on each session launch.
#!/bin/sh
at 1200 #noon
ln -sf [/path/to/new_file] ~/.termux/color.properties
at 1900 #7pm
ln -sf [/path/to/new_file] ~/.termux/color.properties