The default admin scheme for WordPress admin is Gray color scheme. The other available color scheme is the Blue color scheme. Each user can change the color scheme from ‘Profile’ menu on wp-admin. But, when a new user is created or registered, the default color scheme is always maintained unless he/she changes it as per choice.
But what if you are a blue color fan and you wish to change the default admin color scheme to ‘Blue’ i.e. ‘classic’, so that whenever a new user is created, the default color scheme for them would be ‘Blue’.
Include the below few lines of code in your functions.php of your active theme. To get the function.php file, go to Appearance –> Editor –> Theme Functions (functions.php)
At the very bottom of the file put the below code:-
And save the file, its all done.