Hi @Keysersozeey ,
Based on the issue that you mentioned, I have made a test on my side, and the issue is confirmed on my side.
Recently, there are some updates to PowerApps Portals, please check the following article for more details:
https://powerapps.microsoft.com/en-us/blog/power-apps-portals-simplified-themes-in-portals-studio-public-preview/
I think this issue may be related to updates on PowerApps Portal. Currently, the updates have not been applied to all regions. If you want to upload CSS Style file to your Portal, as an alternative solution, you could consider go to Portal Management of your Portal, then switch to "Web Files" option, then click "+ New" button to add a web file.
You need to fill the required fields for the new web file, then click "Save". Then Edit this new added web file, then switch to "Notes" tab, then click "+" to upload your CSS file. If you want to edit the CSS file, you just need to download the CSS file under the Notes Time line, then edit it using your local code editor tool, then remove the previous CSS file under the Notes Time Line, then upload the modified CSS file.
Please check and see if the following blog would help in your scenario:
https://carldesouza.com/how-to-create-and-customize-a-custom-power-apps-portal-for-your-organization/
Best regards,
