Inspect the element of the feedback message and change what you want to change. Here is the CSS for the feedback message.

/*! Widgets - Feedback Message */
.feedback-message{
  -webkit-box-align:center;
      -ms-flex-align:center;
          align-items:center;
  border-radius:var(--border-radius-soft);
  display:-webkit-box;
  display:-ms-flexbox;
  display:flex;
  font-size:var(--font-size-h6);
  font-weight:var(--font-regular);
  left:50%;
  max-width:600px;
  min-width:400px;
  padding:var(--space-base);
}
.feedback-message i{
  -ms-flex-item-align:start;
      align-self:flex-start;
  font-size:var(--font-size-h5);
  margin-top:2px;
  position:relative;
}

.feedback-message-text{
  padding-left:var(--space-base);
}

div.feedback-message-error{
  background-color:var(--color-error);
}

div.feedback-message-info{
  background-color:var(--color-info);
}

div.feedback-message-success{
  background-color:var(--color-success);
}

div.feedback-message-warning{
  background-color:var(--color-warning);
  color:var(--color-neutral-10);
}

Use this to change what you want in terms of size

And change the colour that you want here

Put your changes on the base theme and it will apply the changes to the feedback message

Look at this post, someone did the same

https://www.outsystems.com/forums/discussion/50883/how-to-change-the-feedback-messages-look/

Kind Regards,

Márcio