Site Health: is_writable() check when there are more than 1 theme directories

[pbiron]

While working on the patch to #48199, I discovered the existence of ​register_theme_directory(). Using register_theme_directory() it seems to be possible to have a setup such as:

• WP_CONTENT_DIR/parent-themes/some-parent-theme
• WP_CONTENT_DIR/child-themes/current-theme

where current-theme is a child of some-parent-theme. In such a setup, the current behavior (and that in the patch for #48199) actually checks whether WP_CONTENT_DIR/parent-themes is writable.

2 questions:

1. should the is_writable() actually be checking whether get_theme_root( get_stylesheet() ) is writable?
2. should there be additional checks whether all registered theme directories are writable?