The hovering you can add by nesting the following:
&:hover {
        property: value;
    }
I used your screenshot as example:
.projectReferenceselector{
    .select.form_control{
         background-color: #1c1c1c;
        &:hover {
            background-color: #FFFFFF;
    }
.mx-name-......etc
etc
etc
 }
}
Hi Sidhant,
use the below code for changing the background color for the reference selector.use the class name as ref
.ref{
background-color: #ccc;
select.form-control{
background-color: #ccc;
}
}
it will work.
I was able to add the color to the tabs and hover effect, Thanks Sigrid Nederpelt, Attar Dasthagir and Tim van Steenbergen . The changes were not getting applied because I was using the custom class for tabs inside the reference selector class itself as highlighted below:
So I declared the custom tab class separately, shown below and then the tab color got changed:
(Updated Styling):
 
For Reference selector instead of .select.form-control I used the following code: 
.projectReferenceselector{
//background-color: grey;
.form-control{
background-color: grey;
}
    }