If I’m understanding correctly what your wanting to do you’ll need to override the CSS that is in /wp-admin/css/colors-fresh.css.
To change the default state of a single button you can just add CSS like this
#menu-posts-testimonials-widget {
background: red;
}
this is an example with the testimonial widget plugin you can inspect the element to find out the id of the element and base your CSS off that.