#8333: Use Design to get theme, because theme is changed after constr…

[kirmorozov]

Description

Functionality helps of custom theme per product allows to rollout new theme partially.
But it was broken for some reason.

Solution is applied in production as a patch for 2.2.2.

Fixed Issues (if relevant)

1. Magento v2.1.2 - Choosing a child theme for product page #8333: Magento v2.1.2 - Choosing a child theme for product page
2. Custom theme doesn't apply XML layout updates when applied to a product #7710: Custom theme doesn't apply XML layout updates when applied to a product
3. Category design / theme update not inheriting correctly on product pages #7503: Category design / theme update not inheriting correctly on product pages

Manual testing scenarios

1. Go to product in admin
2. Change theme, save.
3. Selected theme must apply in full

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[orlangur]

Hi @kirmorozov, change looks good to me at first glance, didn't check the logic deeply yet. Would be really nice to cover such case with integration test so that it will be never broken again.

[sidolov]

Hi @kirmorozov , please, take a look to failed tests

[orlangur]

@kirmorozov, closing due to inactivity. Feel free to reach me out anytime later if you wish to continue work on this pull request and it will be reopened.