Remove the Admin Color Scheme Picker

Hides the Admin Color Scheme option on profile screens, keeping a consistent look for every user.

Code

add_action( 'admin_init', function () {
    remove_action( 'admin_color_scheme_picker', 'admin_color_scheme_picker' );
} );

Pair with a snippet that forces one color scheme if you want everyone on the same one.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.