One of the options from Textra v3.22 for the Screen Color, is Light & Night Mode.
If chosen, the screen color will be light during the day and dark at night.
The actual times the screen color changes between light and dark is defined by Civil Twilight.
In a nutshell, it can be explained more simply as follows:
* Textra switches to a dark screen when the center of the sun dips below the horizon, just after sunset.
* Textra switches to a light screen when the sun is 6° below the horizon, just before sunrise.
Can I set the times manually? No, sorry, the intent is that in automatically determining the twilight times, then as the seasons change or you travel, then you don't have to manually change the times, it's automatically managed for you.
The Twilight API is provided as part of the Google Android Firmware. Android asks your network for Twilight times.
If your times initially show 10pm to 6am, that is the default UNTIL textra gets an approx location from your network, this will happen during the day and these times will then reflect that.
Still showing 10pm to 6am? Try connecting to a different WiFi network (also try with WiFi off) and choose the 'Light & Night Mode' option again in Textra Settings to see if Android / Network then updates the times.