It may not be obvious how to style certain dynamic notifications displayed briefly via JavaScript. You can use the following code snippets to style some of these (colors in these snippets are merely examples).
Add/remove from Watchlist[]
/* Watchlist notifications */
.mw-notification {
background-color: #fff !important;
border: 1px solid red !important;
color: black !important;
}
Page save[]
/* Page save notifications */
.postedit-container .postedit {
background: #fff !important;
border: 1px solid red !important;
color: black !important;
}