semantic/src/themes/github/collections/message.overrides

.ui.info.message {
  background: linear-gradient(#D8EBF8, #D0E3EF);
}
.ui.error.message {
  background: linear-gradient(#F8D8D8, #EFD0D0);
}
.ui.warning.message {
  background: linear-gradient(#FFE3C8, #F5DAC0);
}
.ui.success.message {
}