For the convenience of the users, Android has also added a feature in Marshmallow, its flagship OS for the mobile devices, to choose between the dark or light colored themes for the mobile devices. Some people prefer light-colored-themes while others like dark-colored themes as this is a personal matter of likeness. But, this is also a matter of ease and comfort for some of the people. Android users, who work in the well-lit environment, normally, prefer the dark themes but for the other users, the light color is liked. Similarly, at the night time, the light-colored themes are used and in the daylight, the dark-colored themes may be the choice of other people.
One more aspect of choice of dark or light colored themes is the eye sight effects. People with eye sensitively, may be advised by the physicians to use dark, light combinations or so on.
Keeping the convenience of the users, Android offers a feature to choose between the dark or light-colored theme and activate it by just few taps of fingers on screen smart devices. But if you feel some guidance, do look upon the instructions, given below.
Instructions
1: Tap the icon of the ‘Settings’ app, placed at the bottom-right corner of the Home screen of your Android Marshmallow smartphone or tablet to open up the ‘Settings’ screen.
2: At the ‘Settings’ screen, scroll down to the ‘Device’ section and tap ‘Display’.
3: At the ‘Display, screen, see; the current ‘Theme’ status is ‘Light’, i.e. light-colored theme is being applied. This is also evident from the current screen color which is white or light colored but the images and inscriptions / writings are made with black or other colors. 
4: If you want to change the color of the theme, just tap it. It will open up a prompt, named; ‘Theme’. Here, see; currently, the ‘Light’ is activated. The other option of color is ‘Dark’. 
5: If you want to change the theme color, just tap the radio button of the ‘Dark’ and see; immediately, the color of screen will change to dark color. The change will take you, immediately, back to the ‘Display’ screen.
5: At the ‘Display’ screen, see; the status of the ‘Theme’ is also changed to ‘Dark.
That’s all!