Re-apply temporary workaround to generateThemeSite.sh
Re-apply temporary workaround to generateThemeSite.sh

@digitalcraftsman was trying to re-apply this workaround
in commit 3068f640e17a544542be17e4ca842ceb5acfb2a9,
but for some reason that commit contains whitespace cleanup only.
The following commit message was from that commit:

Setting the theme path via the `-t THEME` flag isn't recognized
and still requires to use the HUGO_THEME env variable as workaround.

See gohugoio/hugoThemes#412
See gohugoio/hugo#5061
See gohugoio/hugo#4851

This reverts commit 0b4e98b96f0ee4aee3fd22bc8a7a5e65869d255f.

spf13/hugoThemes,Nytoca/hugoThemes