How to enable modern look & feel for Force.com Site?

Question

We have a Salesforce instance (developer edition). Force.com Site is there. VF page is used by that Force.com Site by Site Guest User. We use VF default components and expected it will have the same look and feel like when we load the same VF page inside of SF instance as an internal user. But for guest user visuaforce pages look different with the same css styles like it was before in 2008.

How can we make it look like for internal users?

UPDATE:

This VP page is used from managed package. There is a piece of code of the VF page:

<apex:page controller="SurveyController" cache="false" showheader="false" action="{!init}" language="{!language}">

<apex:stylesheet value="{!$Page.labs_survey_css}" />
<apex:includeScript value="{!URLFOR($Resource.SurveyForce, 'jquery.js')}"/>
<apex:includeScript value="{!URLFOR($Resource.SurveyForce, 'survey_jui.js')}" />
<apex:includeScript value="/soap/ajax/18.0/connection.js" />
<apex:includeScript value="/soap/ajax/18.0/apex.js" />

        <apex:outputPanel id="selectQuestionType" >
            <span class="prompt">{!$Label.LABS_SF_SelectQuestionType}
                <div id="topPickList">
                    <apex:selectList size="1" value="{!questionType}" id="questionSelect">
                        <apex:selectOptions value="{!questionTypeOptions}" />
                        <apex:actionSupport event="onchange" oncomplete="refreshLayout(true)" 
                                reRender="addQuestion, rowQuestion, rowQuestionPreview, singleSelect, 
                                        singleSelectPreview, multiSelect, multiSelectPreview, freeText, 
                                        freeTextPreview, selectQuestionType, questionBlock" />
                    </apex:selectList>
                    <script>
                        var questionSelect = $(fixId('{!$Component.questionSelect}'));
                    </script>
                </div>
            </span>
        </apex:outputPanel>