Hi Rean,
I regret to hear that it still not works.
You could try this different approach:
Edit the portal app in make.powerapps.com

In editor, we will see a blank page, it doesn't matter because event portal is not supported with power app editor,
we just need to click "Browse website", It will also force the portal website to refresh.

I uploaded a simple CSS file, it take effect immediately by following above process.
Good luck.
(If it is still not working, please check whether spell of Partial Url is correct.)
Regards,
Clofly