Nightlight is a lightweight Open Source app that allows you to toggle your Windows theme between light and dark mode.
Nightlight places an icon down in the systray that permits you to switch themes via a single click. It will be a great asset for users who work long hours behind the keyboard by providing a little eye relief when working in low light or simply having a change in appearance without too much fuss. There is the option, however, to apply Nightlight system-wide. It is available by right-clicking the icon and going into settings where it provides two options that can be used together or separately - the choice is yours.
If you like to switch themes, then Nightlight will be the app you are looking for.