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Test Proper backup and restoration of base_properties on theme change
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Create the browser object we'll be using.

    >>> from Products.Five.testbrowser import Browser
    >>> browser = Browser()
    >>> portal_url = self.portal.absolute_url()
    >>> browser.open(portal_url)

Login as an admin

    >>> self.setRoles(['Manager',])
    >>> self.portal.portal_membership.addMember('manager','secret',('Manager',),())
    >>> browser.getLink('Log in').click()
    >>> browser.getControl('Login Name').value = 'manager'
    >>> browser.getControl('Password').value = 'secret'
    >>> browser.getControl('Log in').click()
    >>> browser.open(portal_url)

Navigate to Theme Control Panel

    >>> browser.getLink('Site Setup').click()
    >>> browser.getLink('Themes').click()

Change to Plone Default Theme

    >>> browser.getControl('Default theme').value = ('Plone Default',)
    >>> browser.getControl('Save').click()

Check if base_properties was backed up
    >>> hasattr(self.portal.portal_skins.custom, 'base_properties.leftskin')
    True

Revert back to Left Skin Theme

    >>> browser.getControl('Default theme').value = ('Left Skin',)
    >>> browser.getControl('Save').click()

Check to see if base_properties.leftskin was restored

    >>> hasattr(self.portal.portal_skins.custom, 'base_properties.leftskin')
    False
    >>> hasattr(self.portal.portal_skins.custom, 'base_properties')
    True
