Changing the background colour of alerts?

Anvil Q&A
4

That’s a good suggestion re adding a role to alerts. We’ll add that to our list.

As a work around you could try something like this

from anvil.js.window import document

def my_alert(*args, role=None, **kwargs):
    try:
        if role is not None:
            alert_dom = document.getElementById('alert-modal').querySelector('.modal-dialog')
            alert_dom.classList.add('anvil-role-' + role)
        return alert(*args, **kwargs)
    finally:
        if role is not None:
            alert_dom.classList.remove('anvil-role-' + role)

then adjust your css to be

.anvil-role-alert .modal-content {
  background-color: black;
}