You can't, a WMS returns a picture of the map that has been rendered by the WMS server. If you want to style a layer in the client you have two choices:

1. request the actual data from a WFS server and style it yourself.
2. Send a SLD file to the WMS server for it to use to style the data.

In both cases this will only work if the server in question supports that functionality.

You can't, a WMS returns a picture of the map that has been rendered by the WMS server. If you want to style a layer in the client you have two choices:

1. request the actual data from a WFS server and style it yourself.
2. Send a SLD file to the WMS server for it to use to style the data. You can check for this option by looking for a line like: <UserDefinedSymbolization SupportSLD="1" UserLayer="1" UserStyle="1" RemoteWFS="1"/> in the getcapabilities response, you need UserStyle to not equal 0.

In both cases this will only work if the server in question supports that functionality.