How to write fallback setting for sphinxjp.themes.* are not found.

conf.py

try:
    import sphinxjp.themecore
    extensions.append('sphinxjp.themecore')
except ImportError:
    extensions = []


try:
    import sphinxjp.themes.bizstyle
    html_theme = 'bizstyle'
except ImportError:
    html_theme = 'default'

extensions.append() するならば先に extensions = [] しておきたい