If you run UWP apps on Win 10 mobile (in dark mode) without any status bar customization, you will likely to see there would a black bar at the place where the system status bar would normally located.
This is caused by the default status bar foreground color, which is set to black. If the device is running in light mode, you will be able to see the status bar.
To make sure that status bar is always visible (regardless of the colour theme of the device), you can customize it through code. You would need to add ‘Windows Mobile Extensions for the UWP’ reference to your project first.
For UWP app running on desktop, you can also customize the title bar of the application using similar method (which you can read more in the source listed below).