Change background color of selected items in "multiselect" dropdown box?

We can simply do with the help of the below CSS:

select option:checked{
  background: #1aab8e -webkit-linear-gradient(bottom, #1aab8e 0%, #1aab8e 100%);
}

<style>
     .select2-container--default .select2-results__option[aria-selected=true] {
        background-color: inherit;
        color: lightgray;
    }
</style>

Add your own style inside the block.