Overwrite Style from dependent picklist

It seems that Salesforce removes all assigned style classes from the dependent picklists. Well, we have to fix it with some javascript tricks. The idea is to re-assign a style class to the dependent picklist after it structure was changed (ie. attributes removed, child elements added).

For that we will use a cool technique with a "MutationObserver" object. Using it we will be able to catch every DOM change on the target element. For example if a "class" attribute removed from the element - re-assign it again.

First define a style class for the picklist. To keep it simple I'll use only a background color "green":

<style>
/* Style class for picklists */
.myListClass {    
    background-color: green;
}
</style>

Then add both picklists to the page. A dependent picklist should be wrapped with some DIV element with an ID and used as a target element for the observer:

<!-- Master and dependent picklists -->
<apex:inputField value="{!acc.MasterList__c}" styleClass="myListClass" />
<div id="selectList">
    <apex:inputField value="{!acc.ChildList__c}"  styleClass="myListClass dependentElement"/>
</div>

Now add a MutationObserver object, a target element (dependent picklist wrapper with an id "selectList") and observer itself.:

<!-- This script should be added AFTER picklists on the page -->
<script>
// Mutation object
var MutationObserver = window.MutationObserver || window.WebKitMutationObserver || window.MozMutationObserver;

// Defining observer 
var observer = new MutationObserver(function(mutations) { 
    // If the myListClass was removed from the list - re-assign it
    if(!jQuery('#selectList select').first().hasClass('myListClass')){
        jQuery('#selectList select').first().addClass('myListClass');
    }
});

// A target object fot the observer
var myNode = document.querySelector('#selectList'); 

// Assigning observer to the dependent picklist
observer.observe(myNode, {
    childList: true,
    subtree: true,
    attributes: true
});
</script>

And the result will look like: