Hi Daniil,
Problem got solved.!!
The problem was, in the project at page level we are setting Namespace=" " in ext:ResourceManger. That's why in the getCssClass function App.combobox was not firing. Now I have set the Namespace in the control level and it is working as expected.
Thanks again, the thread can be closed :)
<ext:ComboBox ID="ComboBox1" runat="server" Flex="1" NoteAlign="Top"
Width="150" Selectable="true" EnableKeyEvents="true"
TypeAhead="true"
QueryMode="Local" SelectOnFocus="true" ForceSelection="true" Namespace="App">
<ext:ResourceManager ID="ResourceManager2" runat="server" ShowWarningOnAjaxFailure="false" Namespace="" />