Created by: cvrebert
See: https://github.com/twbs/bootstrap/blob/5e9b365b2533d245bc992caaad2beca4bc3d701e/scss/_alert.scss#L17 https://github.com/twbs/bootstrap/blob/5e9b365b2533d245bc992caaad2beca4bc3d701e/scss/_alert.scss#L38-L44
