Please anybody help me how to make page block border thin like standard page. 

the border thickness of the visualforce pageblock (Open Action Items & Closed Action Items ) is thicker

than that of the other related lists on this page.

<apex:form>
            <apex:pageblock title="Open Action Items" id="relListPb1">
            <apex:pageBlockButtons Location="top">
                <apex:commandButton action="{!createNewActionItem}" value="New Action Item"></apex:commandButton>
            </apex:pageBlockButtons>
            &nbsp;&nbsp;<apex:outputText value="No records to display" rendered="{!if(OpenTaskSize > 0,'false','true')}" style="font-size:88%"/>
            <apex:pageblocktable value="{!taskRelOpen}" var="tasks" rows="{!ListRowsLimit_open}" id="relListPbt1" rendered="{!if(OpenTaskSize > 0,'true','false')}">
                <apex:column headerValue="Action" >
                <b><apex:outputLink value="{!URLFOR($Action.Task.Edit, tasks.Id,[retURL=URLFOR('/'+$CurrentPage.parameters.Id)])}">Edit</apex:outputLink></b>
                |
                <b><apex:outputLink value="{!URLFOR($Action.Task.Close,tasks.Id,[retURL=URLFOR('/'+$CurrentPage.parameters.Id)])}">Cls</apex:outputLink></b>
                </apex:column>
              <apex:column headerValue="Subject">
                <apex:outputLink value="{!URLFOR($Action.Task.View, tasks.Id,[retURL=URLFOR('/'+$CurrentPage.parameters.Id)])}">{!tasks.Subject}</apex:outputLink>  
              </apex:column>   
              <apex:column headerValue="Assigned To">
                <apex:outputLink value="/{!tasks.OwnerId}">{!tasks.Owner.Name}</apex:outputLink>    
              </apex:column>   
              <apex:column headerValue="Member Name">
                <apex:outputLink value="/{!tasks.WhoId}">{!tasks.Who.Name}</apex:outputLink>
              </apex:column>
              <apex:column value="{!tasks.DateAssigned__c}"/>
              <apex:column value="{!tasks.ActivityDate}"/>
            </apex:pageblocktable>
            <br/>
            <div id="pb1div1" style="{!displayShowMore_open}">
                <apex:CommandLink action="{!showMoreAction_open}" reRender="relListPb1">Show {!showMoreLabel_open} more >></apex:CommandLink>
                    |&nbsp;
              <apex:outputLink value="/apex/RelActionItemsListPage?id={!$CurrentPage.parameters.Id}&dispList=open" target="_parent">Go to list ({!openTaskSize})>></apex:outputLink>
          </div>
    </apex:pageblock>

    <apex:pageblock title="Closed Action Items" id="relListPb2">
        <apex:pageBlockButtons Location="top">
            <apex:commandButton action="{!noAction}" value="View All" rendered="{!if(ClosedTaskSize > 0,'true','false')}"></apex:commandButton>
        </apex:pageBlockButtons>
            &nbsp;&nbsp;<apex:outputText value="No records to display" rendered="{!if(ClosedTaskSize > 0,'false','true')}" style="font-size:88%"/>
        <apex:pageblocktable value="{!taskRelClosed}" var="tasks" rows="{!ListRowsLimit_closed}" id="relListPbt2"  rendered="{!if(ClosedTaskSize > 0,'true','false')}">
            <apex:column headerValue="Action">
                <b><apex:outputLink value="{!URLFOR($Action.Task.Edit, tasks.Id,[retURL=URLFOR('/'+$CurrentPage.parameters.Id)])}">Edit</apex:outputLink></b>
                    |
                <b><apex:outputLink value="{!URLFOR($Action.Task.Delete,tasks.Id,[retURL=URLFOR('/'+$CurrentPage.parameters.Id)])}">Del</apex:outputLink></b>
            </apex:column>
            <apex:column headerValue="Subject">
                <apex:outputLink value="{!URLFOR($Action.Task.View, tasks.Id,[retURL=URLFOR('/'+$CurrentPage.parameters.Id)])}">{!tasks.Subject}</apex:outputLink>  
            </apex:column>
            <apex:column headerValue="Assigned To">
                <apex:outputLink value="/{!tasks.OwnerId}">{!tasks.Owner.Name}</apex:outputLink>    
            </apex:column> 
            <apex:column headerValue="Member Name">
                <apex:outputLink value="/{!tasks.WhoId}">{!tasks.Who.Name}</apex:outputLink>
            </apex:column>
            <apex:column value="{!tasks.DateAssigned__c}"/>
            <apex:column value="{!tasks.ActivityDate}"/>
        </apex:pageblocktable>
        <br/>
            <div id="pb2div2" style="{!displayShowMore_closed}">
                <apex:CommandLink action="{!showMoreAction_closed}" reRender="relListPb2">Show {!showMoreLabel_closed} more >></apex:CommandLink>
                    |&nbsp;
              <apex:outputLink value="/apex/RelActionItemsListPage?id={!$CurrentPage.parameters.Id}&dispList=closed" target="_parent">Go to list({!closedTaskSize}) >></apex:outputLink>
          </div>
    </apex:pageblock>

  </apex:form>