Unable to set className on dropdown?

It seems like one cannot directly assign CSS classes to dcc.Dropdown directly. I was trying to vertically split my view between a label and the dropdown, and I tried to do it with the following css + app.py:

.label {
    display: inline-block;
    width: 15%; 
}                      

.dropdown {                                   
    display: inline-block;
    width: 80%;
}  
import dash
from dash import dcc, html

app = dash.Dash(__name__)

app.layout = html.Div(
        [html.Div('Make your choice', className='label'),
         dcc.Dropdown(['a', 'b'], className='dropdown'),
        ]
)

if __name__ == '__main__':
    app.run_server(debug=True)

This results in the following page:

If I add a style parameter directly to dcc.Dropdown, I can achieve the alignment I want:

import dash
from dash.dependencies import Input, Output
from dash import dcc, html

app = dash.Dash(__name__)

app.layout = html.Div(
        [html.Div('Make your choice', className='label'),
         dcc.Dropdown(['a', 'b'], className='dropdown', style={'width': '80%', 'display':'inline-block'}),
        ]
)

if __name__ == '__main__':
    app.run_server(debug=True, port=8051)