Correct the values for theme_root and stylesheet when the values passed to the...

Correct the values for theme_root and stylesheet when the values passed to the WP_Theme constructor have a directory appended to the theme root when it should actually be prepended to the stylesheet (when the theme is in a directory of themes inside a theme root). see #20313.

git-svn-id: https://core.svn.wordpress.org/trunk@20316 1a063a9b-81f0-0310-95a4-ce76da25c4cd