When using ipywidgets the way to create a button is basically as follows:

import ipywidgets as widgets
layout_btn = widgets.Layout(width='200px')
mybtn = widgets.Button(description='load',
                       disabled=False,
                       button_style='', # 'success', 'info', 'warning', 'danger' or ''
                       tooltip='this will happen:...',
                       icon='',
                        layout=layout_btn)

in the Layout object several things can be defined: see here:

Follow the link for more details about the style attribute.