Are there good tools for XFCE that switch themes at a particular time?
You mean themes as themes or wallpapers? If you mean themes as themes I have never seen such a tool.
AutomaThemely? You can also add proper commands to cron.
It can be done on other DE’s, see these scripts:
If someone figures out how to do the same for Xfce let me know and it can be added to the wiki there.
EDIT: I figured it out with the help of @mr_glitch. It’s now live in the wiki.