Someone on chat suggested it would be nice to have a dark mode on the Lines app - I responded with some tricks I use and thought it would be useful to have that info easier to find over here in the forum.
NDR - Anyone else think a dark mode setting would be a nice option to have available in the touring plans app? It would be nice to have to tone down the screen when using the app in dark spaces, night or indoor queues.
FWIW: Even though there’s no app setting for Lines dark mode: On my iPad, if I triple-push the main physical button on the bottom of the iPad it turns everything into dark mode, including Lines.
Also, if you have an Android phone you can force dark mode even for apps that don’t have it built in.
You have to enable and go into Developer mode and select the “Force Dark Mode” option - then when you use the “dark mode” option on the Android menu it should force Lines into that mode too.
This page has an easy step by step on how to do it. Good luck! 
This Hidden Samsung Galaxy Feature Enables Dark Mode On Every App
In case the page ever gets taken down:
Samsung Galaxy phones have a “Force Dark mode” feature that’s hidden within the developer options menu which can affect all Android apps installed into the device when switched on, including those that don’t outwardly offer dark mode functionality. Before you can access it, though, you need to enable the “Developer options” menu, which is hidden by default on most Android devices. Follow these steps (per Samsung):
Go to “Settings” and select “About phone” or “About device.”
Hit “Software information.”
Tap “Build number” seven times, then key in your PIN, password, or pattern to access the “Developer options” menu.
You’ll know if you followed the steps correctly if “Developer options” is now listed in your phone’s settings. Depending on what Android device you have, you may need to go into the “General” submenu to get to it. Once you have access to the hidden function, you can now force dark mode on all your installed apps — provided that you have at least Android 10 installed — by doing the following:
Go to “Settings” and hit “Developer options.” If it’s not there, you may need to go to “General” to get to it.
Scroll down to the “Hardware accelerated rendering” section and switch the “Force Dark mode” toggle on.
To ensure that dark mode is properly applied everywhere on your Samsung Galaxy device, make sure you also have dark mode enabled through the Quick settings panel by swiping down on your main screen using two fingers and tapping the dark mode or night mode icon until it is glowing.