Adding CSS to JS files

I ahve a number of JS files with message boxes in them.  Is there a way to add a global CSS style to them.  Or, do i have to add the style to each one indivdually? This is an example of a JS file that I have.

$(function() {

    $('select[name=myClients]').on('change', function() {

        let selectedValue = $(this).val()

        updateView(selectedValue);

    });



    $('#cancel-sessions').submit(function(e) {

        e.preventDefault();

        if (!confirm('Are you sure you want to cancel the selected sessions')) {

            console.log('returning');

            return;

        }

        $('#form-message').hide();

        $.post(this.action, $(this).serialize(), function(resp) {

            updateView(0);

            if (resp.msg) {

                $('#form-message').html(resp.msg).show();

            }

        }, 'JSON')

    })



    function updateView(selectedValue) {

        $.ajax({url: 'bookedSessions.php', success: function(result) {

            $('#results').html(result);

        }});

    }

})

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