From the case I understand that you when you upload a new CSS file you see that the hosted UI does not use the new CSS. Please let me know if I misunderstood the issue.
The likely issue in this case is that your app client is using client-level settings for CSS and you are uploading the new CSS file to the app client default hosted UI customization. I am attaching the following documentation that goes over specifying app UI customization settings here (1). Once you locate the app client settings for the app client that you wish to modify you can check to see if the hosted UI customization is using default settings or if it is using client-level settings. If the app client is using client-level settings then you would have to edit the CSS in the app client similar to how it is done in steps 5-8 in the documentation. Editing the app client default settings for the Hosted UI as is done in step 4 would not edit the CSS for app clients that use client-level settings rather than default settings.
If you have any follow-up questions I would be more than happy to answer them. I hope you have a great rest of your day!
Brilliant, I had forgotten about client-specific customization. Will take a look, thank you!
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