Hi Everyone.
I want to add colors to LWC standard Lightning datattable. Please let me know how to add. 
<div class="slds-m-top_small slds-m-bottom_xx-large" >
                        <c-custom-type-component
                            class="POdataTable" 
                            key-field="Id" 
                            data={POrecords}
                            columns={columns} 
                            onrowselection={selectedRecord}  
                            selected-rows={preSelectedRows}
                            onsave={handleReturnQuantity}
                            draft-values={saveDraftValues} 
                            onclick={removeInlinePopover}
                            oncancel={clearSelecteData}
                            min-column-width="120"
                            onrowaction={getPopUpInfo}
                            suppress-bottom-bar={supp}
                            onpicklistchanged={picklistChanged}
                        ></c-custom-type-component> 
                    </div>

-----JS----
{label:'PO#', fieldName:'PO_Number__c', type:'text',class : "POdataTable", innerWidth:120, cellAttributes: 
        { class: 'slds-text-color_success slds-text-title_caps slds-color__background_gray-5'}
    },-------- so on columns
-------CSS-----
.THIS {
}
.THIS .POdataTable thead th span { 
background-color: #2a5596; 
color: white;
}