Cannot resolve symbol 'Theme' in styles.xml (Android Studio)
Another solution that worked for me with Android Studio 3.1.2:
delete these from build.gradle if you have them:
'com.android.support:appcompat-v7:27.1.1''com.android.support:design:27.1.1'
then sync,then undo delete,then sync again.
I encountered this after updating to Android Studio 3.1.
None of the other answers listed here worked for me, however when I switched my appcompat support lib version to the latest 28.0.0-alpha1 then back to 27.1.0 that it was on before, it worked.
Seems the new version of Android Studio lost connection to where the library was synced to and just needed to have it reset.
No need to change anything.
Just press small button on top "Sync Project with Gradle Files"