Migration from OBIEE 11g to OBIEE 12c don't affect default skin/style
Change default skin/style It's not a good idea, You can have many problems after that.
It's recommended that you create your custom style
Answering your questions...
1 - Your default come from analytics.ear, you must create your custom style to change it according you want (see above link)
2 - Verify your instanceconfig.xml to see which style is calling
3 - All config files are under skin/style folder, but while you don't create yours (custom skin/style) you cant change it
4 - No, this migration doesn't affect that
Were you able to resolve this? We have the exact same issue.
Has anyone been able to resolve this? I have the exact same issue as well. I have even tried to configure custom style sheets after installing patch 22140759 and following Doc ID: 2131220.1 and still have remnants of blafp within my alta style.
Might it be that you've changed the style for you dashboards (in dashboard properties) and those are pointing to "blafp" instead of "Default (Alta)"? I'd have issues with the skin and pointing the dashboards to "Alta", saving them, and then pointing them back to "Default (Alta)" solved my issue.
Did you try cleaning your browser cache?