theme: Uniform the style of the dialog warnings

theme: Uniform the style of the dialog warnings

The end session dialog is now using a new style for the warning messages
but this style is not re-used by other dialogs that have messages with a
similar role.

So be consistent, and use same style for all the dialogs that are
showing a warning or an info message, and in particular
 - Keyring dialog error messages
 - Polkit dialogs error and info messages
 - Conflicting dialog warning message

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3897>