There is no fix to this right now except manually overwriting the CJui CSS files after putting them in a name scope as pointed out by Mihai P.
Or you can do this, the CSS files which loads in the end wins the class precedence. So you can write your code something like :-
$this->widget('zii.widgets.jui.CJuiAutoComplete',array(
'name'=>'city',
'source'=>array('ac1','ac2','ac3'),
'options'=>array(
'minLength'=>'2',
),
'htmlOptions'=>array(
'style'=>'height:20px;',
),
));
$this->widget('yiiwheels.widgets.datepicker.WhDatePicker', array(
'name' => 'datepickertest',
'pluginOptions' => array(
'format' => 'mm/dd/yyyy'
)
));
Call the autocomplete first so that the CSS files from YiiWheel's Datepicker are loaded in the last.