Revert "Disable Theme Engine"

This reverts commit 46d27a53c9b5028ece8455c44f925f6d5bdeddd3.
Revert "Remove theme_settings"

This reverts commit 7ecf94e3efcc0564c4ee99d55c0c3108837be559.
Revert "Remove Theme Engine"

This reverts commit c3440439a07325fa2f5d22222b30c5a2c6799deb.