Custom CSS in iOS mobile App

I want all circle avatar pictures - like in almost every other modern communications app. This seems to do the trick via custom CSS in the Layout Settings:

.rcx-avatar__element {
    border-radius: 1.25rem !important;

It works very well on the browser app (even mobile) and the desktop app, but not on the iOS App. Am I missing something, or is there no way to apply the custom css to the mobile app?