Installing

pip install sphinx-bulma-theme

Configuring

import sphinx_bulma_theme

html_theme = "bulma"
html_theme_path = [sphinx_bulma_theme.get_html_theme_path()]

html_theme_options = {
    'show_topbar': True,
    'topbar_class': 'is-info',
    'topbar_logo_class': 'image is-32x32',
}

See also

List with all theme options