Alert blocks provide an area to add text and a selector to choose the “background“ color. These use Bootstrap alerts styling.
See an example of this content type
Instructions
Go to Structure > Block layout > Custom block library (tab) > Add custom block (button) > Alert
Give the alert a memorable Block description
The Block description will not display on the alert, so it is a good practice to give it a memorable title that is relevant to where it will reside to make it each to fine in admin screens.
Provide the Body text of the alert
Select the Type
The Type is what dictates the alert background color
Click Save
To place the alert
Go to Structure > Block layout
Click the Place block button next to the announcement region
Find your block in list (look under Custom in the Category column)
Click Place block on right
Change title if you want to, and normally your should uncheck the Display title checkbox.
Then, using normal block placement tools choose ‘where’ you want it.
@TODO link to block placement help
Click Save block