This isn't super Pythonic - try this:
```python
theme_id = options.get('theme_id')
if theme_id
    return '/admin/themes/%s/' % theme_id
else:
    return '/admin/'
```

`None` will evaluate to `False` for conditionals, so you don't need to explicitly check that the value is `None` unless you're worried about there being a different value in there that also counts as `False`. I can't think of a time when a `theme_id` would be `False`, so I think you probably don't need that check.