Here is a notification from the default Xfce Debian GNU / Linux install:
... produced by xfce4-notifyd package, here is an alternative
... this second example (New updates) is produced by notification-daemon-xfce package.
If you really like a square dialogue then you might prefer the results from notification-daemon-xfce.
But wait - I like the ability to 'click off' a notification, so I should go with notification-daemon-xfce? Maybe.
Xfce4-notifyd and 'click off' feature:
Hover over the notification and an X will appear for you to 'click off' the notification so that it disappears.
Prefer to have notifications stay around until you manually 'click off'?
Increase 'Disappear after' to be 300 seconds, which means notifications stay on screen for 5 minutes.
It is now up to you to 'click off' notifications yourself pretty much.
The theme 'Smoke' is selected above, that gives the grey with rounded edges, that you see in first screenshot example 'Device is now safe to remove'.
( That colour is probably #4D4D4D which some folks label 'grey 30' )
But my notifications options screen does not have all those options?
If you have notification-daemon-xfce installed then your 'Notifications' options would instead look like this:
Where are all these notification settings anyway - which menu?
Xfce Menu -> Settings -> Xfce4 Settings Manager
Notes and Further reading:
About xfce4-notifyd package:
Themable using the GTK+ theming mechanism. Visually appealing: rounded corners, shaped windows
About notification-daemon-xfce package:
Clone of notification-daemon but without GNOME dependencies. It permits doing passive popups notification on the user desktop to notify about events.
If you are running Ubuntu, then do be aware that notification mechanisms for some applications might well be patched by Ubuntu to fit in with their particular way of doing things.