EXTJS4:如何改变grid某一个单元格的背景颜色

Ext.onReady(function () {
Ext.widget('grid', {
title: 'Users',
store: {
fields: ['name', 'email', 'online'],
data: [
{ 'name': 'Lisa', "email": "lisa@simpsons.com", "online": true },
{ 'name': 'Bart', "email": "bart@simpsons.com", "online": false },
{ 'name': 'Homer', "email": "home@simpsons.com", "online": true },
{ 'name': 'Marge', "email": "marge@simpsons.com", "online": true }
]
},
columns: [
{
header: 'Name',
dataIndex: 'name',
renderer: function (value, meta, record) {
meta.tdCls = record.get('online') ? 'user-online' : 'user-offline';
return value;
}
},
{ header: 'Email', dataIndex: 'email', flex: 1 },
{ header: 'Online', dataIndex: 'online' }
],
width: 400,
renderTo: 'output'
});
});

.x-grid-cell.user-online
{
background-color: #9fc;
}
.x-grid-cell.user-offline
{
background-color: #ffc;
}